2.1 – These systems do not use the rear and centre speakers, leaving only the
sub-woofer and two front speakers.

5.1 – Three front speakers, positioned to the left, right and centre at the front of the room, but also includes two rear speakers positioned behind the listener and a subwoofer positioned elsewhere in the room.The lower bass notes that come from the subwoofer are less directional than the higher frequencies, positioning is less critical than the placing of the satellite speakers.

7.1 – Takes the basic 5.1 layout and adds two more rear speakers. However, the majority of Blu-ray discs are only mixed for 5.1 audio so the addition of these two extra speakers has no significant effect. Some 7.1 discs are available but in small numbers. 7.1 is for those who want the ultimate system.