When Magic 2010 was released a few fundamental changes to the way Magic had been played were made. One of those changes was that combat damage no longer used the stack. Prior to this change, creatures in combat would assign damage, this damage would go on the stack as one item on the stack (assuming that first strike was not involved), and when this item resolved all combat damage was dealt as it was assigned. The important wrinkle here is that after combat damage is assigned and before it resolves players could play spells and abilities. If a creature in combat left the battlefield it would still have its damage dealt. This proved to be very unintuitive to newer players and created a ‘trick’ of the rules that gave a considerable advantage only relied on a player having a little more experience.
As mentioned, this was changed when Magic 2010 was released. Combat damage does not use the stack, and it is dealt immediately after it is assigned. Anything that you intend to play before a creatures deal or receive combat damage now have to played before combat damage is assigned.