The community version is what was previously known as "cooker". Which was a testing/unstable release (alledgedly). I don't know if the actual "cooker" thing still exists, because when "they" made the changes, they made it so that "community" was more towards stable than cooker, then changed the name.
So at the moment, the initial release is "community", and then a couple of months down the line, when any possible snags have been "ironed out" to the satisfaction of mandrake (or mandriva as they're becoming), they then release "Official". Which, in theory, is stable. Well really, there'll probably always be a few minor bugs, but that's what the bug reporting system is for.
If you don't have much experience with linux, stick to the Official releases, if you don't mind too much, and what something that's likely to be more up to date, then community might be the way to go.