Introduction
Daoism (spelled Taoism in the older romanization system) is an indigenous Chinese religious tradition in which reverence for the Dao (see below), is a matter of ultimate concern. Daoism does not have a founder or principal scripture. Different adherents, communities and movements revere different individuals and scriptures, although the Daode jing (see sec.3) is probably the most central and influential scripture. Daoism flourished in China along other two influential doctrines, Buddhism and Confucianism, and it's safe to say that it represents the more free, imaginative and poetical side of the Chinese culture. The various popular religious practices present throughout all China, even if greatly influenced, are usually not strictly identified with Daoism.

 

Scientific Denomination
Both “Daoism” and “Taoism” refer to the same Chinese religion; they are both pronounced with a “d” sound. “Taoism” derives from an earlier way of approximating the sound of Chinese characters into alphabetic script. “Daoism” derives from the more recent Pinyin romanization system, which is the official system of the People’s Republic of China. This is the preferred form.
Even today, some old-fashioned world's religions' textbook utilizes the misleading distinction between so-called  philosophical Daoism and so-called  religious Daoism, the use of these categories should be taken as a sign of ignorance and inaccuracy. The easiest solution to this problem is to  replace “philosophical Daoism” with “Classical Daoism", which indicates, from the very beginning, a communities of thinkers which scholars categorize as equally "religious". See also intercultural and interdisciplinary information below.

Place and time of Origin
As with many ancient religious traditions, an exact date is impossible to determine. Daoist ideas and early writings long precede organizational structures. In fact, From the fourth to second centuries BCE there formed the early "mystical" lineages, master-disciple communities, which some scholars refer to with the labels "Classical Daoism" or "Proto-Daoism". It is associated with famous texts like the Daode jing and Zhuangzi (see sec. 3). The beginning of Daoism as an organized religion begins in second century CE and  extends to roughly the seventh century.

Current distribution among world population.
The number of Daoists is difficult to estimate, due to a variety of factors including defining Daoism. For example, if people practicing Chinese popular religious practices, which  have been influenced by Daoist tradition, are taken into account, the numbers of followers reach 400 million. More balanced estimates for the number of Daoists worldwide range around twenty millions. Geographically, Daoism flourishes best in regions populated by Chinese people: mainland China, Taiwan, Malaysia, Philippines,  Singapore, and various Chinese diaspora communities. Daoist literature and art has influenced the cultures of Korea, Japan, and Vietnam. There are also small numbers of people that identify themselves as Daoists in the Western world.

 

Main doctrinal tenets: the Daoist concept of Dao
In the Chinese context, Dao is not a concept exclusive of Daoism. However, for Daoists, Dao represents the sacred or ultimate concern. From a Daoist perspective, there are four primary characteristics of the Dao: (1) Source of everything; (2) Unnameable mystery; (3) All-pervading sacred presence; (4) The process (Nature) which sustains the Universe. The Dao is impersonal and ineffable. Through a spontaneous, impersonal process, the Dao move from primordial undifferentiation to differentiation (the manifest world). Daoists thus emphasizes emanation and immanence. There is no necessary distinction between the Dao as unnameable mystery and its various phenomenal expressions. The various gods that Daoist worship are seen  as purer manifestations of the Dao. Far for being a eternal Absolute  behind a world of change, Dao is  much more  the impersonal natural  and cyclical pattern, the vital flux of the cosmic and earthly order. Broadly understood, the point of a Daoist way of life is to cultivate alignment and attunement with the Dao.

Source's analysis

Source n° 1
The Chinese character for Dao
When used in the discussion of traditional Chinese thought (not only in Daoism) Dao is maybe the most important concept. It may refer to the "Way of Heaven", the "Truth of the universe", or the correct way to live and behave. It can also refer to a way to rule over or lead others.
Etymologically speaking, in the character 道, on the left and bottom there is the part 辶  which means "walking". The character 首  on the top-right is a pictogram depicting the head. It refers to the head, a leader or chief, or being first or at the beginning. Thus, 道 (dao) expresses the concept of a continuous, active pursuing the right direction (in spiritual cultivation, in the society, in politics, ecc.). Daoism give this Dao a peculiar metaphysical and religious characterization (see above).

Source n°2
Map of China
As it can be easily seen,  major Daoist sacred sites  are mostly situated in the plains between the Wei and Yangtze Rivers, where the Chinese civilization begun its development and expansion. This further enlighten the Chinese character of Daoism and its historical relationship with ancient Chinese territories. Other, more peripheral provinces, in fact, tend to have bigger concentrations of Buddhism and other religions' followers.

Intercultural and interdisciplinary information
(History, Philosophy)
The distinction between so-called “philosophical Daoism” and “religious Daoism” is a modern Western fiction, which reflects colonialist and missionary sensibilities. In fact, the Chinese term for “religion” and "philosophy" are an invention coined in the late 19th century. "Religion", as it is commonly defined today in modern, secularized societies (largely a product of European “Enlightenment” thinking of the 17th and 18th centuries) is incompatible with the way religious thought and practice were construed in traditional China. Quite apart from relegating “religious thinking” to a specialized domain, the dominant strands of thought China conceived an integrated cosmos in which heaven and earth, gods and humans, the living and the dead were all interconnected. In this conception there was no clear separation between sacred and profane. Also the concept of philosophy, in the sense of purely intellectual, disembodied thinking fails to adapt to Daoism. Although there are aspects of Daoism that are “philosophical,”  the term “philosophical Daoism” fails to consider the centrality of embodied practices and communitarian life.