If you're new to Bitcoin, and the cryptocurrency market in general, Satoshi Dice is probably the first gambling site you happen to stumble over. If you look at their site, satoshidice.com, you get to read a long and lavish story about who "Satoshi" is. But the fact of the matter is that Satoshi Dice is just a simple numbers game. And it's quite easy! All you have to do is decide on how much you want to bet, how good of odds you want, and that's it! Their system will then "roll a dice" and decide if you win or lose. Almost instantaneously. Even better, if you lose, you get some of your money back.
But be aware that even thought some bets may seem favorable, like the "90%" bet, you won't double up on your money on easily. You'll just have to trust in the ghost of Satoshi!