Base classes are kinda like templates of properties and methods which are used to create the regular classes that share them, for example Part and MeshPart both share stuff like Position or Size but MeshPart also has MeshId.
It’s done so that they don’t need to copy/paste the same functions into a bunch of different classes.
I don’t think there’s a list of just base classes, but you can generally see the inherited classes of something under its properties on devhub like so:
Alternatively this site might make it easier to browse base classes: Roblox API Reference
I did make sure to look at Documentation and API Reference, But its still Difficult to find a specific one as there are Hundreds (potentially thousands) of Classes.
I don’t think you’ll find anything better than the site I linked, it lists the inheritance tree on the side in a nice way
This is parts & models for example