Tor: How it works
Basic Tor overview: what Tor provides, why it matters, how it works.
Installing, configuring, and running Tor.
The design document (published at Usenix Security 2004) gives our justifications and security analysis for the Tor design:
The specification aims to give developers enough information to build a compatible version of Tor:
Look at the slides from the 21C3 talk. For something more obsolete, look at the slides from the Codecon 2004 talk, or listen to the audio from the talk.