Why does a balloon 'stick' to a wall when one has rubbed it on one's head?

When the balloon is rubbed on your head, it becomes electrostatically charged (this is because electrons are transfered to/from the balloon from/to your hair). When you bring the balloon, which now has an excess of positive or negative charge, towards the wall, the charges on the wall that are the same as the charge on the balloon (positive or negative) are repelled into the wall (because like charges repel). This leaves an overall oposite charge, to the charge on the balloon, on the wall. Since oposite charges attract, the balloon is attracted to the wall and 'sticks' to it.