When referring to surround sound, the classifications are generally notated with two numbers seperated by a decimal. The first number represents the number of audio channels or speakers, and the last number represents the number of subwoofer channels.

5.1 = 5 channels and 1 subwoofer.
7.2 = 7 channels and 2 subwoofers.
8.1 = 8 channels and 1 subwoofer.