Secular clergy, regular clergy, orders
The difference between secular clergy and regular clergy is one of Catholicism's most important feature. Secular clergy lives in the saeculum, i.e. in the world: this clergy serves in parishes. These clerics are integrated into a hierarchy, manifested in degrees obtained through a sacrament, the ordination: these holy orders are deacon, priest, bishop.
Regular clergy consists of men and women having taken their vows. Some may have been ordained, but not necessarily: they are often lay people having a more religious life. As the name suggests, regular clergy live by a rule (Latin regula) governing their community. In Eastern Christianity, every monastic institute possesses and observes their own rule. For its part, the Latin Church chose to associate each one of these particular way of life with a different order. Monks are given a single founding rule (the Rule of Saint Benedict) and each new interpretation of said rule causes the creation of a new monastic order. Furthermore, some laymen and clerics choose to form congregations formed around a rule other than Saint Benedict's: which is why they are not monks, but belong to another kind of religious order. Some, like the Carmelites, may be closer to monks because of their contemplative life, but others, unlike monks, choose to be operate in contact with the faithful.

Beginnings of monachism
The emergence of monasticism is tied to Saint Anthony of Egypt (252-356), although there is evidence of this lifestyle before him. He decides to take the Gospel literally and leaves everything to follow Christ. At the beginning, he simply lives as an eremite (or hermit, or anchorite): i.e. he retreats from the world to live in poverty and chastity, and to pray. His experience is emulated, and soon disciples come to him in order to practice eremitism nearby.
These groups do not gather in communities. Cenobitism (living together) was created by Saint Pachomius (ca. 292 - 348). He decides to gather within an enclosure men desiring to experience this retreat from the world, but are unable to live as hermits. These monks live in small individual rooms (or cells), but work, eat and pray in a common area.
Monasticism spreads throughout the Roman East and, soon after, in the West. Palestine in the fifth century is where the last form of monasticism, the Lavra, appears. The first of such settlements were founded by Saint Chariton (died 350), but Saint Euthymius (377-473) and Saint Sabbas (439-532) are the ones who actually organized and structured the Lavra. The monks lead an eremitic life and on Saturdays gather in community buildings where they bring their handiwork, share a meal and attend mass.
The monks garner popularity: their demanding lifestyle becomes almost synonymous with holiness. But they constitute a problem for the Church because they are not part of the ecclesiastical hierarchy. Each community is led by a ‘Father’ (Syriac ‘abba’, hence the term ‘Abbot’) and follows its own rules. Only few monks are ordained as priests or deacons, which places them out of the scope of the bishops’ authority. Gradually though, canon law brings monasteries under their respective bishops’ authority.
The life led in monasteries gradually progressively gains a clearer structure. Saint Basil of Caesarea (329/30-379), who founded monasteries in Cappadocia, writes a few organisational recommendations that do not constitute a rule per se, but serve as an inspiration for the founders of many monasteries. Among them, Saint Benedict of Nursia (ca. 480-547), who incorporates many of Basil’s principles when he drafts his rule for the monastery of Monte Cassino. Saint Augustine (453-430), who led a monastic life before being elected bishop, writes a set of recommendations — including some aimed at a community of women led by his sister — that are now known as the rule of Saint Augustine.

Western monastic and religious orders in the West
Latin monasticism gains its structure thanks to Benedict of Aniane (ca. 750-820). He is convinced that the various rules followed by monasteries are derived from the Rule of Saint Benedict which balances work, prayer, and study. Benedict of Aniane then ‘reconstructs’ the ideal rule. In 816 and 817, Emperor Louis the Pious (r. 814-840) gathers two synods in order to enforce this rule in every Latin monastery. From then on, every monk must follow the rule of Saint Benedict: the Benedictine Order is born. These synods also create the Canon Order, which includes priests attached to a cathedral and priests obeying the rule of Saint Augustine.
The rule of Saint Benedict is the object of two main new interpretations causing the emergence of two new monastic orders. Cluny Abbey, founded in 909, emphasises praying and as well as celebrating Mass. It becomes the heart of a group of abbeys and priories (subsidiary houses of an abbey), thus forming an order.
Cîteaux Abbey was founded in 1115 in order to go back to the balance offered by Saint Benedict's rule: Cistercians criticize the Cluniac wealth and idleness, and stress collective poverty and labour. Cîteaux serves as an example for other new abbeys and soon find itself leading a new monastic order. In the 17th seventeenth century, faced with what some see as a slackening discipline (the order allows other income than what results from the monks' work), a reform movement arises from the idea to go back to Saint Benedict's rule. Starting in 1662, the monastery of la Grande Trappe emerges as the heart of this yearning for reform — which is why the monks of the Order of Cistercians of the Strict Observance are called ‘Trappists’.
In 1084, the founding of the Grande Chartreuse by Saint Bruno of Cologne marks the creation of a new contemplative monastic order that however does not follow the rule of Saint Benedict. Initially, Saint Bruno settles in the Chartreuse Mountains with a few companions to live as a hermit, but the success of his endeavour results in the creation of many similar congregations and ultimately leads to the creation of a new religious order.
During the 12th and 13th centuries, major changes in the medieval West cause the emergence of numerous religious orders. In 1120, at the Bishop of Laon’s request, Saint Norbert of Xanten sets up a community of Canons in Prémontré. Their task is twofold: praying and guiding the faithful. This founding is quickly emulated, and the order is established in 1126. Right from the start the Premonstratensians (or Norbertines) are organised in a three-fold structure: an order for men, an order for women, and a third order for lay people wishing to join this spiritual way of life without going as far as taking their vows.
In the following century, problems caused by growing towns and by the emergence of heresies questioning the role of a wealthy Church too caught up in the worldly, and perceived as corrupt because of it, arise. The mendicant orders, named like this because of their commitment to poverty, provide a way to manage these problems. The Order of Friars Minor (acknowledged in 1210) was founded by Saint Francis of Assisi to devote lives to the ideal of poverty: the brothers make their living exclusively from working and from receiving charity. From 1220 on, the Order focuses on intellectual and pastoral activities. Changes in the Order are responsible for a number of divisions. The Order of Preachers (approved in 1216) was created by Saint Dominic in the context of the conflict with the Cathars. Saint Dominic aims to fight heresy by preaching, but also through his example of a poverty contrasting with the Church’s wealth as it is criticized by the heretics. The voices of Franciscan and Dominican preachers quickly gain a large audience. These preachers also make for a better guidance of the faithful, especially in cities, where the parish network is often lacking. These mendicant orders are used as a model when Palestinian hermits are organized in the Order of Mount Carmel. During the 16th sixteenth century, the order expands thanks to the changes, emphasising prayer and poverty, established by Saint Teresa of Avila and Saint John of the Cross. All these mendicant orders also possess a threefold structure: a male branch, a female branch, and a third branch, the secular order, made up of lay people.
The Crusades and the Spanish Reconquista lead to the creation of religious-military orders. The model of these order in the Templar Order, founded in 1120 and endorsed in 1139. They can be traced back to groups of knights committing themselves to the protection of Church and of Church property. Hugues de Payns turns this desire into vows. The knights become a religious order subject to a rule, but since they are not ordained, they are not included in the ban on weapons for the clerics. The success of the Templar Order leads to the militarisation of several charitable orders which cared for and sheltered the pilgrims. These are the Order of the Knights of Saint John (or Order of Hospitallers), founded in 1180 and endorsed in 1113 and the Order of Brothers of the German House of Saint Mary in Jerusalem (Teutonic Order), established in 1191 and endorsed in 1198. In Spain, the confrontation between Christians and Muslims causes the creation of military religious orders such as the Order of Calatrava, founded in 1164, or the Order of Santiago, endorsed in 1170.
States become stronger and military religious orders do not survive this evolution. The Order of the Temple is abolished in 1312 after a major scandal. In the 16th century, the Spanish orders get tied to the Spanish Crown before becoming honorary orders. The Hospitallers and the Teutonic Order survive by claiming territories as their own and governing them: the Hospitallers in Rhodes and then in Malta, the Teutonic Order in Prussia. The latter is secularised during the Reformation but is reinstated in Austria purely as a charitable order. The Knights Hospitaller are chased off Malta before reinventing themselves in 1879 as a charitable order, commonly known as the Order of Malta.

Eastern monasticism
Byzantine monasticism is heavily influenced by the prestige of the eremitical model. Monasteries are often founded as a result of hermits agreeing to organize the community of disciples that has gathered around them. Another distinctive feature is the existence of urban monasteries playing a significant part in public welfare.
At the end of the eighth century in the monastery of Stoudios (Constantinople), Saint Theodore the Studite attempts to rehabilitate cenobitism by focusing on the value of work. Stoudios becomes one of the most important monasteries in the Empire, but the monasticism's lack of structure lessens its impact. The founding of the Great Lavra Monastery on Mount Athos (963) marks the revival of Byzantine monasticism. Saint Athanasius chooses to settle on Mount Athos (Northern Greece) where a great many hermits are living. He soon decides to build a monastery on the model of the Lavra: most monks live as a community and only a select few, deemed worthy by the abbot, lead an eremitic life in tucked away cells. With the support of the emperor, Athanasius makes a number of adjustments, turning the monastery into a large landowner possessing a very effectively managed estate which can sell its surplus production. Other monasteries, built after the same model, are established on the Holy Mountain. Some are built for monks from all over the Orthodox world: Iviron (Georgia), Zograf (Bulgaria), Saint Panteleimon (Russia), Hilandar (Serbia). The success of athonite monasticism encourages other monasteries to better manage their many assets. Smaller monasteries are absorbed by larger ones and become dependencies (metochia). These economic achievements allow monasteries to provide public assistance by distributing food or by administering hospices and hospitals. Prominent lay people no longer hesitate to found monasteries (doubling as charitable institutions). So they transfer all or part of their assets as they now know that the monasteries will go on after their death. These donators indeed believe that the monks' prayers and charitable actions will help the salvation of their souls. Such is the case of the Monastery of the Mother of God, founded at the end of the eleventh century by Gregory Pakourianos in Bachkovo (Bulgaria) and still active today.