"The experimental distribution is a special distribution. It is not a full distribution in the same sense as `stable' and `unstable' are. Instead, it is meant to be a temporary staging area for highly experimental software where there's a good chance that the software could break your system, or software that's just too unstable even for the unstable distribution (but there is a reason to package it nevertheless). Users who download and install packages from experimental are expected to have been duly warned. In short, all bets are off for the experimental distribution."The experimental packages are automatically removed once you upload the package in unstable with a higher version number.
"The unstable distribution (sid)
The code name for Debian's development distribution is "sid", aliased to "unstable". Most of the development work that is done in Debian, is uploaded to this distribution. This distribution will never get released; instead, packages from it will propagate into testing and then into a real release. Please note that security updates for "unstable" distribution are not managed by the security team. Hence, "unstable" does not get security updates in a timely manner. For more information please see the Security Team's FAQ.
"sid" is subject to massive changes and in-place library updates. This can result in a very "unstable" system which contains packages that cannot be installed due to missing libraries, dependencies that cannot be fulfilled etc. Use it at your own risk!"
What does the testing directory contain?
Packages are installed into the `testing' directory after they have undergone some degree of testing in unstable. They must be in sync on all architectures where they have been built and mustn't have dependencies that make them uninstallable; they also have to have fewer release-critical bugs than the versions currently in testing. This way, we hope that `testing' is always close to being a release candidate. "Debian “testing” distribution"
The code name for the next major Debian release after sarge is “etch”.
This release started as a copy of sarge, and is currently in a state called “testing”. That means that things should not break as bad as in unstable or experimental distributions, because packages are allowed to enter this distribution only after a certain period of time has passed, and when they don't have any release-critical bugs filed against them. Please note that security updates for "testing" distribution are not managed by the security team. Hence, "testing" does not get security updates in a timely manner.