A maniac is someone, who play far to many hands, and who also tend to play them aggressively. He is basically fighting (to) hard for every single pot, and because his range is wide, a lot of his bets and raises are bluffs. Maniacs are also sometimes called "aggro fish". To play against maniacs you need to have a solid bankroll and be patient. Since they are typically not interested in folding, you should reduce bluffing and especially bluffing without equity, but you should widen your value range.
For instance it is often not a good idea to 3-bet a maniac before the flop with 65s, as you would do against a good player, because he is just going to call with his whatever, and his whatever is mostly ahead of 65s. But you can perhaps 3-bet AJ offsuit for value, which you would have folded against a good player.
After the flop its kind of the same thing, and you should also look to do a fair bit of bluff catching against maniacs. If for instance you open KK, the maniac call you in position, and flop comes JJ5 rainbow, then check to him and let him barrel off with his whatever. Sure from time to time he will have a J, and you will have huge egg all over your face. But most of the time he will have absolutely nothing and be drawing nearly dead against you.