As a general rule there are three (3) Debian Distributions: Unstable, Testing & Stable.
- STABLE – This is the latest official release of the Debian GNU/Linux distribution. This is stable and well tested software, which changes only if major security or usability fixes are incorporated.
This release is recommended for the casual user. It has a strong system for fast reactions to security exploits. It will feature the latest linux kernel as of the package release date (It is usually old by the time you get to look at it!). I’m pretty sure Debian policy stipulates any packages with an “important” bug which is not resolved in 10 days, is not accepted into stable. This policy is taken to the extreme and treated very seriously (There is some talk about a time when Apache was nearly thrown out for not conforming to the strick standards of support required for entry into the stable distribution!).
- TESTING – This area contains packages that are intended to become part of the next stable distribution. There are strict criteria a package in unstable (see below) must obey before it can be added to testing. Note that “testing” does not get the timely security updates from the security team.
These packages are “in the wings,” waiting to become “STABLE.” It will contain packages without release-critical bugs. Packages will have the same version number across all architectures they are present in, and they will have spent two weeks or longer in “UNSTABLE.”
- UNSTABLE – This area contains the most recent packages in Debian. Once a package has met our criterion for stability and quality of packaging, it will be included in testing. “unstable” is also not supported by the security team.
Packages in unstable are the least tested and may contain problems severe enough to affect the stability of your system. Only experienced users should consider using this distribution.
These packages may also be referred to as bleeding edge. This area is where developers are still hard at work. There are probably still debates about feature set should be included and how long it will take to make it work correctly.
Some of the information provided was obtained here.