The orishas are the gods of Santeria, the beings that believers interact with on a regular basis. Each orisha has its own distinct personality and has a wide variety of strengths, weaknesses and interests. In many ways, therefore, understanding an orisha is like understanding another human being.
OlodumareThere is also a more removed being known as Olodumare, who created the orishas but later retreated from his creations. Some describe the orishas as being manifestations or aspects of Olodumare.
Olodumare is the source of ashe, which all living things must have in order to survive and succeed, including the orishas. Olodumare alone is self-sustaining, not needing ashe to be provided by another source.
Humans and orishas, however, provide ashe to each other through a variety of rituals. The best source of ashe is in sacrificial blood, which is why animal sacrifice plays such a prominent role in Santeria. Humans provide ashe through blood or other ritual actions, and the orisha becomes a conduit of ashe from Olodumare to the petitioner to assist in the petitioner's endeavors.
Old World and New WorldThe number of orishas varies among believers. In the original African belief system from which Santeria originates, there are hundreds of orishas. New World Santeria believers, on the other hand, generally only work with a handful of them.
In the New World these beings are commonly seen as family: they marry each other, give birth to others, and so forth. In that sense they work more like Western pantheons like those of the Greeks or Romans.
In Africa, however, there was no such familiarity between orishas, in part because their followers were not strongly connected. Each African city-state had its own single, patron deity. A priest could only be dedicated to that single orisha of the city, and that orisha was honored above all others.
In the New World, Africans from many city-states were thrown together into common slavery. It made little sense or practicality for a slave community to to focus on a single orisha in that scenario. As such, the orishas came to be regarded as roughly equals as cultures mixed. Priests were trained to work with multiple orishas instead of being exclusively dedicated to a single one. This helped the religion to survive. Even if a priest of one orisha died, there would be others in the community trained to work with that same orisha.
The PatakisThe patakis, or stories of the orishas, are not standardized and are frequently contradictory. Part of this comes from the fact that these stories come from a variety of different African cities, each of which had their own ideas about the nature of the orishas. This trend is encouraged by the fact that each Santeria community today remains independent of other communities. There is no expectation that each community would function exactly alike or understand the orishas in exactly the same way.
As such, these stories give multiple origin stories for the orishas. Sometimes they are depicted as once-mortal figures, often leaders, who were elevated by Olodumare to divinity. Other times they are birthed as higher beings.
The purpose of these stories today is to teach lessons rather than relate some literal truth. As such, there is no concern about the literal truth of these tales or the fact that tales my contradict one another. Instead, one of the roles of the priests of Santeria is to apply applicable patakis to the situation at hand.
Catholic MasksThe orishas are equated with a variety of Catholic saints. This was a necessity when slave-owners refused to allow slaves to practice African religion. It is understood that the orishas wear many masks in order for people to better understand them. Santeros (Santeria priests) do not believe that the orishas and the saints are identical. The saint is a mask of the orisha, and it does no work the other way around. However, many of their clients are also Catholic, and they understand that such clients better identify with these beings under the guise of the saintly counterparts.
Read more about individual orishas: