The Debian GNU/Linux FAQ
Chapter 5 - The Debian FTP archives

5.1 What are all those directories at the Debian FTP archives?

The software that has been packaged for Debian GNU/Linux is available in one of several directory trees on each Debian mirror site.

The dists directory is short for "distributions", and it is the canonical way to access the currently available Debian releases (and pre-releases).

The pool directory contains the actual packages, see What's in the pool directory?, Section 5.10.

There are the following supplementary directories:

/tools/:

DOS utilities for creating boot disks, partitioning your disk drive, compressing/decompressing files, and booting Linux.

/doc/:

The basic Debian documentation, such as the FAQ, the bug reporting system instructions, etc.

/indices/:

The Maintainers file and the override files.

/project/:

mostly developer-only materials, such as:

project/experimental/:

This directory contains packages and tools which are still being developed, and are still in the alpha testing stage. Users shouldn't be using packages from here, because they can be dangerous and harmful even for the most experienced people.