As of 2.8.1 Beta. WordPress has a User Modell containting, User, Roles and Capabilities. It has been implemented as classes (since around WP 2.0). Next to the classes there are functions in the global namespace and global variables.
This page is to document the current implementation with as much strict data as possible. Official documentation is Roles and Capabilities.
Capabilities is the overall term for WordPress capabilities. Capabilities is the data the the decision of what a user is allowed to do is based on. Compare to Permissions in RBAC.
All capabilities the user has, including individual and role based.
Capabilities of a concrete User (becomes obsolete).
Capabilities of a concrete Role.
Currently it is unknown to me what the concept is, but some information was available: Meta Capabilities are a certain type of capability. Meta Capability can be of:
As written in IRC meta capabilites are dynamic capabilities in the terms of checking if a user can edit a post with a specific id. Infact - as confirmed in IRC - this is a static capability in the name of 'edit_own_posts'.
Currently it is unknown to me what the concept is.