Some deities are servants of more major ones rather than just being their own. They share most of the attributes with the Gods, but they have less power and more leeway. They can reach through the barriers of the Second Breaking to an extent, but they are limited. They can support fewer clerics.
With all this said, the minor gods still hold immense power compared to most other beings. And many worship them in addition to the major gods, whenever the minor one is applicable.
Some would consider Archdevils and Demon Princes to be minor deities as well. Theologians have given this much thought without agreement. Others usually don't care about the distinction.