Debian always has at least three releases in active maintenance: "stable", "testing" and "unstable".
The "stable" distribution contains the latest officially released distribution of Debian.
This is the production release of Debian, the one which we primarily recommend using.
The current "stable" distribution of Debian GNU/Linux is version 5.0.4, codenamed lenny. It was released on January 30th, 2010.
The "testing" distribution contains packages that haven't been accepted into a "stable" release yet, but they are in the queue for that. The main advantage of using this distribution is that it has more recent versions of software.
See the Debian FAQ
for more information on what is "testing"
and how it becomes "stable"
The current "testing" distribution is squeeze.
The "unstable" distribution is where active development of Debian occurs. Generally, this distribution is run by developers and those who like to live on the edge.
The "unstable" distribution is called sid.
sid will never be released. Packages go from sid into testing after a variable period of time, usually around 7 to 10 days.
Debian sid FAQ - http://wooledge.org/~greg/sidfaq.html
Debian - Wikipedia, the free encyclopedia - http://en.wikipedia.org/wiki/Debian
There is also Experimental, which is not a "release" like the others. Experimental is used for packages which are still being developed, and with a high risk of breaking your system. It's used by developers who'd like to study and test bleeding edge software. Users shouldn't be using packages from here, because they can be dangerous and harmful even for the most experienced people.
Once "stable" is released it does not receive new packages other than for security fixes and some bug fixes.