Debian consists mainly of these:
Stable: current Stable is named Squeeze.
Testing: current Testing is named Wheezy
Unstable: always known as Sid
Generally speaking, testing is performing in practice as a very 'stable' distro and those sources you must use if you are aiming in to a kind of a rolling distribution. Stable's disadvantage is that you have to wait for about two years in order to get new packages -only security updates in the meantime.
Mint Debian points testing. What' s the meaning of these? It receives all those upgrades and, when some day Wheezy goes Stable, Mint will stick to the new Testing regrardless of its new name.
If you want to chance your sources to stable, the safe way is this: install from January's cd, change the sources and THEN perform an update/upgrade. Changing sources in a working system backwards (wheezy to squeeze, or sid to wheezy) is not recommended. Cause, you ll end up with a strangely mixed-up system, librareries from here and there etc.. The other way (squeeze to wheezy) is okay, it only means (a ton of) upgrades.
(the following from wikipedia)
stable, currently aliased squeeze, is the current release that has stable and well tested software. Stable is made by freezing testing for a few months where bugs are fixed to make the distribution as stable as possible; then the resulting system is released as stable. It is updated only if major security or usability fixes are incorporated. After Debian 6.0, new releases will be made every two years. Stable's CDs and DVDs can be found in the Debian web site.
testing, currently aliased wheezy, is what the next major release will be and is currently being tested. The packages included in this distribution have had some testing in unstable but they may not be completely fit for release yet. It contains more modern packages than stable but older than unstable. This distribution is updated continually until it enters the "frozen" state. Security updates for testing distribution are provided by Debian testing security team. Testing's CDs and DVDs can be found on the Debian web site.
unstable, permanently aliased sid, repository contains packages currently under development; it is updated continually. This repository is designed for Debian developers who participate in a project and need the latest libraries available, or for those who like to "live on the edge", therefore it will not be as stable as the other distributions. There are no official CDs/DVDs because it is rapidly changing and the project does not support it, although CD and DVD images of sid are built quarterly by aptosid. Additionally, the other two distributions can be upgraded to unstable.