Religion is a system of beliefs held by a group of people involving the practice of certain beliefs and customs. Beliefs vary greatly, the practice of religion can rule over some people's lives while for others it is but a part of everyday life or a belief they uphold.
The numerous religions found throughout the galaxy vary widely, but there are many common themes. Many religions feature a god often held responsible for the creation of and/or guidance of a civilization or the entire universe. Many religions also feature an afterlife, commonly including places for the good and evil, heaven and the underworld, or hell. (TNG comic: "Captain's Pleasure", et al.)
It is also common for religions to feature stories of creation and significant figures in the history of religion, often intended to teach a moral. For some, these stories reflect an actual history, whilst for others, those who follow the religion and those who do not, they are sometimes seen more as tales as guidance, not to be taken as the literal truth. For instance, in the Bajoran religion the Prodigal was a significant figure, and as such the namesake of one of Bajor's moon, Baraddo, to the many Bajorans this was historic fact and valuable to their religion, but the Tellarite Gann considered it more of a fairy tale whilst Keiko O'Brien interpreted the Bajoran religion more along the lines of a mythology. (DS9 novel: Devil in the Sky)
The Prophets, the gods of Bajoran religion, feature in the religious beliefs of several races, though each race's interpretation of them is quite different. To the Bajorans the Prophets guide and protect Bajor, they are gods to be revered. To the Eav'oq, the Siblings (as they call the prophets), are considered more spiritual equals, whilst to the Ascendants the True are beings to aspire to be, they seek the Fortress of the True in the hope of being judged worthy of joining their gods. (DS9 novel: Rising Son)
Certain artifacts often hold religious significance, sometimes from actual mystical properties of the artefacts, such as the Bajoran Orbs, or for symbolic or historic reasons, such as the Tellarite Scroll of Eternal Feasting. (DS9 episode: "Emissary"; DS9 novel: Devil in the Sky) For the Skorr, it was the Soul of Skorr. A sculpture made of indurite; it acted as a katric ark containing the life energy of their most hallowed prophet, Alar. Lara, a hunter/scout, invoked the "Seven Gods", a pantheon in their religion as to why Tchar took to acts of crusadership over it. (TAS episode: "The Jihad")