An "undervote" is a special kind of non-vote. It is the kind of non-vote where the voter failed to detach the chad from the ballot. In particular, if you went to the polls, got a ballot, did nothing to it, then dropped it in the box, you have just "under-voted". Your ballot has no vote on it, therefore it is an "undervote". (Sometimes, of course, the people who attempt to count "undervotes" think they can spot a little faint impression, or "dimple" as they call it, which - THEY THINK - indicates that the voter who turned in this ballot MAY HAVE meant to push that chad all the way off. This is completely subjective after-the-fact mind-reading, however.)
Now, for "overvote". An "overvote" is a ballot on which two (or more) chads are detached for the same office (for which only one choice can be valid). In other words, if you punched the chads for both "Gore" and "Bush", you have "overvoted". This, too, is a special category of Non-Vote, because after all if you vote for more than one candidate your vote is disqualified, since there's no real way to determine who you meant to vote for. (On the other hand, the people who engage in counting things like "overvotes" seem to use the following criterion: "Any ballot with both the Gore Hole and someone else's hole punched out, must REALLY be a Gore vote.")
So there you have it. Two different categories of Non-Votes: that is all that they are.
Hope this helps. ;)