A maniac is a player that bets first and looks at his cards later. Now, the definition I have used there may be a bit extreme. I may be getting a bit carried away. This is what tends to happen to players when they run into a maniac.
There will be varying degrees but the term 'maniac' is generally used to describe a player that is regularly betting aggressively without a hand. If handled correctly both you and your bank balance will thoroughly enjoy the company of maniacs at your table. However, it is important to note that a solid player can become a poor player and lose money with the arrival of a maniac.
At this point you may ask – "How is a maniac going to beat an otherwise solid player over the long term?". The answer is – he won't. However, it is the other good players that are likely to take you're money should he fall into a common trap when confronted with a maniac.
To analyse this situation thoroughly you need to consider the average players emotional reaction to the arrival of a maniac. Let's see. You have developed a solid game and are playing against a typical mix of opponents. Most are solid players but there are a couple that are playing loose. You may be up or down on the session but you understand that as long as there are inferior players by playing a solid, disciplined game you will win over the long term.
Enter the maniac. After a few hands it is apparent that this player is not following any form or disciplined poker strategy. He is here to gamble! A typical reaction would be to look at his stack, look at yours and then mentally picture the two stacks merging together. You are excited at the opportunity that has presented itself. This is all fine as long as it doesn't cause you to become a loose fish yourself.
Your play really should not alter much at all. Many experts would suggest that you should play even tighter when sharing the table with one or more maniacs. The theory being that when the betting is loose and aggressive and you enter the pot with a substandard hand the size of the pot can cause you to stay to the end with the second, third or fourth best hand.
My advice is to stay patient and play your normal game. The only variation may be to check and call to the end with a mediocre hand when you are head up against a maniac when you may have folded against an average opponent. The exception to this rule should is when other previously reasonable players start jumping into hands with mediocre hands in an effort to get some of the action before the maniac runs out of money. In this case, you should play a bit tighter only entering the pot with very good hands. This can not only allow you take the maniacs money over the long term but also the money of the otherwise reasonable players who are now throwing chips around left right and centre.
So when a maniac joins your table the average pot size and your pulse rate are likely to increase. If you want the same to happen to your bank balance you just have to remain patient.