Alright, this is really getting old (excuse the pun). All of this information is available at Debian.org, the Debian wiki, and the internet at large, but somehow people still spout inanities about the Debian distributions.
Debian always has at least three active distributions: Stable, Testing, and Unstable. These names do not mean what you think they mean. Unstable and testing do not necessarily crash all the time. The names refer to the distribution holistically.
Stable is currently codenamed "Woody" and was released on July 19th, 2002. Nothing will be added to this release other than security updates. Installing new packages and kernels on Stable defeats the entire point of running it in the first place and makes baby Jesus cry. You can do it but if you do it a lot, start tracking a different release. Typically Stable is used for critical servers.
Testing is where packages go once they have undergone some degree of testing in Unstable. It's supposedly always on the verge of being a a release candidate. Packages here are more current than Stable and less erratic than Unstable. However, not all the bugs have been ironed out yet and Testing does not enjoy official support from the Debian security team (security.debian.org). Eventually Testing will be "frozen" (no new packages unless they contain release-critical bug fixes) and be declared Stable. At this point the previous Stable distribution gets archived and the new Stable distribution is released. So, when Sarge finally becomes Stable, Woody will be "put back in the cupboard" with Potato and Slink and Sarge will become Debian 3.1 (probably; I'm not sure how version numbers are determined). Typically Testing is run on non-critical servers or desktops.
Unstable is always called Sid. As well as being the boy next door who destroyed toys, SID is also (unoffically) an acronym for "Still In Development". Any package here may have critical bugs and may conflict with other packages. For me using Unstable on the desktop has resulted in nothing more than minor glitches, however, there have been severe bugs in Unstable. Probably the most famous of which is when Pam broke, meaning no one -- not even root -- could log in. YMMV. Typically Unstable is run on desktops or development servers.
There is also Experimental which is not a distribution at all. Packages here are inconsistant and of alpha release quality. There is no automated build process for these packages so invariably they're only i386. Most Experimental packages have at least some known release-critical bugs. Users are strongly discouraged from using packages from here; typically these packages are developer fodder only. Caveat aptor.
So what does this all mean for Joe Linux?
Stable -> Server@work.
Testing -> Desktop@work or *@home.
Unstable -> LatestAndGreatestDesktop@home.
Experimental -> Never@ever.