It's actually pretty different, you really notice it when you play it-I'm not a fan of 5 a side, but love futsal!
To start with, it can go over head height, the ball can go out of bounds, and you can enter the keeper's areas. The goals are also taller (about the size of hockey goals). The ball is about 25% smaller than an 11 a side ball, and 30% harder.
As the ball can go out of bounds, this actually reduces the size of the pitch a fair bit from 5 a side, which means that when it's played well (that Italy game they just showed wasn't the best) you really have to hammer passes to each other as they need to get through tiny gaps. Whereas 5 a side is about a lot of hustle, the best futsal teams are all pass and move-the difference between futsal and 5 a side is a bit like the difference between Arsenal and Bolton-both can be really effective when they play well, but only one is beautiful!