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 addtional code needs to be written, usually when you are skipping something.