The official models released by Stability AI and their partners are called base models. Some examples of base models are Stable Diffusion
1.4,
1.5,
2.0, and
2.1.
Stick with the base models if you are starting out.
Custom models are trained from the base models. Currently, most of the models are trained from v1.4 or v1.5. They are trained with additional data for generating images of particular styles or objects.