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

Debian 6.0.5, or squeeze.  Access this release through dists/stable
Debian 6.0.5 was released Saturday, 12 May 2012.

Testing, or wheezy.  Access this release through dists/testing.  The
current tested development snapshot is named wheezy.  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.