Trac rests on top of Subversion, and provides

• a web interface to the repository, for easy code browsing (and studying differences made in sources over time),
• a Wiki, for easy setting up web pages about the project, and
• a Ticketing system for keeping track of problems that should be addressed.

The strong point of Trac is that all the above are well integrated. You can refer to everything eveywhere in the system, eg refer to a source code line (of a specific revision) in a ticket, or to a ticket in a Wiki page. See http://devel.se.wtb.tue.nl/trac for a list of Trac's that are currently being used. Near the top-right is a Help/Guide link (in red) with the online manual of Trac.