Schema

Database definitions. Makes changes to the schema database when this is required (e.g. exchange install).

Domain Level:

PDC Emulator

Handles password updates, group policy updates, time updates and acts as the master browser.

Relative Identifier (RID)

Provides SIDs (Security Identifiers) for users, computers, groups etc.

Infrastructure Master

Keeps track of who is in what group. Should not be on a server that is also a global catalog, unless all DCs are global catalogs.