• NOTE: Description of how the code works (when it isn't self evident).
• XXX: Warning about possible pitfalls, can be used as NOTE:XXX:.
• HACK: Not very well written or malformed code to circumvent a problem/bug. Should be used as HACK:FIXME:.
• FIXME: This works, sort of, but it could be done better. (usually code written in a hurry that needs rewriting).
• BUG: There is a problem here.
• TODO: No problem, but additional code needs to be written, usually when you are skipping something.

At least this is how I was taught about these tags. Basically the first two (NOTE and XXX) are used for information and no action is required. While the last three (FIXME, BUG and TODO) do require action. HACK is somewhere in between (and hardly ever used I think?).