This directory, dists, is the canonical way to access the distributions.
Each distribution can be accessed by name or state from here.

oldstable, or lenny         - the released Debian 5.0.9.
stable, or squeeze          - the released Debian 6.0.4
oldstable-proposed-updates  - possible updates to Debian 5.0.  See the README there.
proposed-updates            - possible updates to Debian 6.0.  See the README there.
squeeze-updates             - important updates to Debian 6.0
testing, or wheezy          - the development version of the next release.
unstable, or sid            - untested candidate packages for future releases.