The difference is in the number of speakers. A 3.1 system has 3 speakers and subwoofer
(Right Left, Center) (3) and a Subwoofer (1) 3.1
5.1 system has 5 speakers and subwoofer
(Right, Left Center, Rear Right Rear Left) (5) and a subwoofer (1) 5.1
the first number is the speakers then the dot and the second number is the subwoofer.
therefore a 7.1 system has 7 speakers and a subwoofer
the biggest difference between 3.1 and 5.1 is 5.1 is Surround sound. 3.1 is not. and with 7.1 it gives that extra chanel front and rear to heighten the sound experience