the first number before the period is the number of speakers, the second number after the period is the number of subwoofers.
the most common are 2.0, 2.1, 5.1 and 7.1. less common but used are 3.0, 3.1, 4.0 (quadraphonic), 6.1 and 9.1 along with double subwoofer styles such as 5.2, 7.2, 9.2 etcetera.
anything 2.0 or 2.1 is stereo sound (left and right only).
anything 5.1 (technically 4.0 but that is hardly used) or greater is called surround sound and if the speakers are placed right will give you a sense of directional sound
since your other thread was in reference to computers and since you posted in the computer peripherals forum i'm assuming you are talking about computer speakers here. the 3 most common choices are 2.0, 2.1...