Debian Project News recently linked to a helpful infographic of Debian versions. It doesn't include anything about backports, volatile/updates, or contrib, main, and non-free, but it's a great resource nonetheless. The main point i would disagree on is its suggestion that testing is appropriate for general users (and it seems that a number of the commenters on that post agree). I always recommend stable for all users unless there is a really good reason to do otherwise.
Wouldn't it be great if we had similar infographics for Ubuntu (including universe/multiverse) and Red Hat/Fedora/CentOS?