Religion
A religion can be defined as a system of beliefs, values, principles, philosophy, mythos, and rituals that deal with the ultimate concern of a person or society. Common (but not universal) elements of religion include organized worship (possibly including a place of worship, commonly called a temple, church, or shrine), beliefs regarding the supernatural, and respect for a high figure (who may or may not be divine).
Organizing Religions
Organizing religions is a difficult task, as theologians differ on where the differences lie. For example, while "Christianity" describes a set of common beliefs in God, Jesus, and the afterlife, there are vast differences in the various churches of Christianity.
As an example, the Baptist faith, the Catholic Church, Mormonism, and the Jehovah's Witnesses all base their faith on the Bible, but take different views on such things as organization, the necessity of proselytization, the rituals that have to be observed to be a devout Christian, the will of God, and the nature of evil.
However, there are some common divisions that many scholars agree to:
"Abrahamic", monotheistic religions believe in ONE creator god, prophetes, and an afterlife. Although Judaism is traditionally believed to be the oldest of the three main brances, Rabbinical Judaism developped parallel to Christianity. Both stem from the earlier Israelite religion and Zoroastrianism -- Christianity owing a lot to Greek and (later) Roman thinking. Christianity got a clear form until through the ecumenical councils if the fourth century, Judaism by the redaction of the Babylonian talmud. Abrahamic religions include Catholicism, Protestantism (which is separated into hundreds of brances), Orthodox Christianity, Oriental Christianity, Mormonism, and Islam (which has several major branches).
Buddhism is ultimately derived from the life and teachings of Siddhartha Gautama (c. 600 BCE), although his teachings did not displace, but instead absorbed, the philosophies of the regions it spread to, providing for a continuence with other earlier faiths. The three main branches of Buddhism are traditionally regarded as Theravada, Mahayana, and Vajrayana, although some scholars speak of a "fourth Buddhism" coming into existance as the religion is studied in areas politically, geographically, and linguistically distinct from Buddhism's Indian origins. It also played an important role in Daoism and Confucianism; however, these generally absorbed Buddhists philosophies, not the supernatural or ritual elements of Buddhism.
Hinduism is a group of distinct but intimately related religions, made possible through the practice of henotheism. It is sometimes said that there as many forms of Hinduism as there are Hindus; however, the most-worshipped trinity of gods includes Brahma, Vishnu, and Shiva. It had a strong influence on other Indian religions, including Jainism and Sikhism.
Neopaganism (or "New Age") is a term that was coined to describe the modern trend of new religions which have a strong belief in the supernatural. Some religions strongly reject the label as it has been used derisively by scholars of other faiths, while some religions accept the label proudly. These religions are very diverse, from doctrinal to philosophical; some seem to have been created-whole cloth, while others are explicitly based on another religion. Some of the best known Neopagan religions include Asatru, Raelism, Scientology, Tenrikyo, and Wicca. Unitarian Universalism is sometimes considered a Neopagan religion, and sometimes a Judeochristian religion; it merits individual study.