A mod for Blizzard games such as StarCraft, Diablo II and Warcraft III. It disables fog of war for the game in question.
The main problems with maphack are to do with its use on Battle.net. A few antisocial bastards use it to gain stupidly high rankings on the ladders. These people generally have no understanding of how maphack works and use it simply to give them the rights to yell n00b into the chatbox.
Blizzard tries to maintain a hard stance on map hacking. Anyone found using maphack will get their CD-Key banned at the next patch. Every patch also invalidates current maphacks in circulation.
Using maphack is generally considered an unwise thing to do as a large proportion of files floating around which claim to be maphacks are in fact trojans of one sort or another. Even after you find a working maphack, there are still the issues of banning and social rejection to contend with.