The CVS Repository Exploring perspective lets you bring code from CVS into your workspace. If another developer changes the source in CVS while you're working on it, the IDE helps you synchronize with CVS and resolve any conflicts. You can also choose to automatically notify the CVS server whenever you start working on a file. The CVS server then notifies other developers who work on that file as well. Finally, the CVS Repository Exploring perspective lets you check your modified code back into CVS.
Since the CVS Repository Exploring perspectives is a core Eclipse feature, you'll find complete documentation in the Eclipse Workbench User Guide.
Subversive isn't included with the Eclipse Platform, and to be useful within Eclipse, it must be used in conjunction with separately-licensed connectors.