Each “world religion” is actually a classification of multiple distinct movements, sects, divisions and denominations and none are single, unified, monolithic organizations. The diversity within these groupings varies. Hinduism, for example, is often described as a collection of very different traditions bound by a geographical and national identity. So broad is this religious “umbrella” that it includes polytheistic, tri-theistic, monotheistic, pantheistic, non-theistic and atheistic traditions.
The Babi & Baha’i traditions, on the other hand, are probably the most unified of the classical world religions. They are almost entirely contained within one very organized, hierarchical denomination, the Bahai Faith, based in Haifa, Israel. But there are small schismatic groups, such as the Arizona-based “Orthodox” Baha’is, Azali Babis (probably defunct) and four or five others.
All adherents of a single religion usually share at least some commonalities, such as a common historical heritage and some shared doctrines or practices. But these rules can’t be pushed too far before being overburdened by exceptions. A listing of doctrinally and organizationally meaningful divisions or denominational “branches” (such as Catholic, Eastern/Orthodox Christian, Sunni Islam, Shiite Islam, Evangelical Christian, Mahayana Buddhism, Theravada Buddhism, etc.) would clearly be useful, but that is the subject of a different list: Major Branches of Major World Religions.
From a sociological and historical perspective, most religions have arisen from within existing religious frameworks: Christianity from Judaism, Buddhism from Hinduism, Babi & Baha’i faiths from Islam, etc. For the purposes of defining a religion we need to have some cutoff point. Should Sikhism be listed as a Hindu sect (as in many older textbooks), or as a world religion in its own right?
To manage this question we have chosen to use the most commonly-recognized divisions in comparative religion texts. These definitions are primarily sociological and historical, not doctrinal or theological and have been presented here in order of the number of world wide adherents (with kind permission from http://www.adherents.com)
Following is a link to a calendar of religious festivals from different faith paths and spiritual traditions: http://www.reonline.org.uk/supporting/festivals-calendar/