5.1 is where you have 5 speakers and a sub
Two behind you, left and right Two in fornt of you, again left and right One in the middle in front of you - This is the center speaker used mainy for dialogue and the .1 is the sub woofer - this can be anywhere, as frequencies this low are onmi-directional (your ears cant tell where they are comming from!)
6.1 Has another center speaker behind you 7.1 has the same as 5.1, but also another two, left and right beside you, 8.1 has the lot!