What Is Pascal's Wager?

Pascal was a seventeenth Century mathematician who basically came up with a 'wager' to make people believe in God.  He argued that if God does exist then you should believe in him because then you would have everlasting life and a jolly good time in heaven.  If God does not exist, then all you have to lose by believing in him is cutting out some of the 'naughty' bits of a finite life.  Even if you think that the chances of God existing are say 1 in 10, the only rational thing to do is to believe in God.  Because if you do not you would lose more after death than you had lost in life !  So it is a win win situation for God.  
The argument is clearly very reliant on there being a heaven, because if God exists, but there is no heaven then it may be irrational to believe in God.  But we cannot prove there is no heaven, so Pascal would argue that all we can do is believe in God.....