Hedge funds are investment vehicles that have avoided being heavily-regulated because they only accept money from government-certified "accredited investors." Their investment strategies are secret and unconventional. Some of them deliver floods, while the average one doesn't do that well.
Hedge funds provide liquidity, which makes it easier to trade; and conduct arbitrage, so that prices are muscled into line before they get totally irrational. (The latter doesn't always work though.) Meanwhile, since they have fewer safety nets, hedge funds are more aggressive about monitoring their own risk. They're better than banks in that respect, which are much more heavily leveraged and which have a history of being supported by the government when they mess up. Hedge funds have never been bailed out by the government like banks have time and again.