I think the main difference that is a little confusing, is that in a Re-entry game, you have the opportunity to buy back into the game only when you go broke, this of course has a Time limit, which is determined by the Blind level.
In a Rebuy Game you have the same opportunity with the added bonus that you can also purchase an Add-on at any time during the game regardless of how many chips you have, but again there is a similar time restriction, and that is determined by the Blind level and break level, So once you reach the allotted Break you will be given a chance to Add-on, but once the game restarts you have no option for either.
It's very important to read the Tournament information before you register so that you know whether it is a Re-entry or Re-buy with Add-ons, because with some Games you are limited to one Re-Entry or Re-Buy, but with others there could be unlimited Re-Entries until a certain Break Level is reached.
Hope this clears the mud a little.