5.1 = 1 center, 1 left front, 1 right front, 1 left rear surround (behind you, to the left), and 1 right rear surround (behind you, to the right).
7.1 is the same as 5.1, except the two extra speakers go directly to the left and right of you
here is a graphic of a 7.1 system...it's the same with 5.1 except without the two speakers directly to the side of you:
