Google Analytics Segments (for API v3)
Segments - Feature Reference
Explains the terms Users, Sessions, Hits and their relation to segments.
Gives examples on conditional segments and how these conditions are applied to segments.
Core Reporting API - Segments
Explains dynamic segments, more complex conditions as well as operators for advanced conditions.
Gives information on which metric can be applied to Users, Sessions and Hits.
Google Analytics API - Default Segments List
Provides a list of default API segments and their IDs.
Google Analytics - Segments: list
Documentation on how to get a list with all default and customized segments that may be used by an account.