See http://www.debian.org/ for information about Debian GNU/Linux.
Three Debian releases are available on the main site:

Debian 5.0.5, or lenny.  Access this release through dists/stable
Debian 5.0.5 was released Saturday, 26 June 2010.

Testing, or squeeze.  Access this release through dists/testing.  The
current tested development snapshot is named squeeze.  Packages which
have been tested in unstable and passed automated tests propogate to
this release.

Unstable, or sid.  Access this release through dists/unstable.  The
current development snapshot is named sid.  Untested candidate
packages for future releases.

Older releases of Debian are at http://archive.debian.org/debian-archive

--- Other directories:
doc             Debian documentation.
indices         Various indices of the site.
project         Experimental packages and other miscellaneous files.
tools           Tools for creating boot disks and booting Linux.