Surround sound is the name of a number of technologies for sound reproduction. At the start, sound was reproduced using a single channel (called mono), later there were two channels, left and right (called stereo). Surround sound extends this idea, by adding more channels. There are speakers that surround the listener. The speakers are grouped, and each group has its own channel. There are different setups ranging from 2 or 3 speakers/channels in the front, and two in the back, up to a setup with 24 speakers (known as 22.2)
With surround sound, channels are treated differently, to be able to create special sound effects. In classical theatres, surround sound is not used; they may also have different speaker with different channels, but all channels are treated the same.