The 5.1 channel setup was the first surround sound system design. The "5" denotes the five different channels of sounds built into the system. Each channel of sound comes from a different direction. The directions are: front center, front left, front right, left and right. The ".1" denotes the low frequency effects channel of sound. This LFE channel is sent to the subwoofer. All DVDs are compatible with the 5.1 setup.
The 6.1 channel setup is the exact same as the 5.1 channel setup but with an added channel of sound coming from the rear of the room. The 6.1 setup is popular but not as common as the 5.1 setup. The 6.1 setup creates a more enveloping sound as sound is now coming from all four directions of the room instead of only three. Most, but not all, DVDs are compatible with the 6.1 setup.
The 7.1 channel setup is the newest of the three surround sound systems and the least commonly used. The 7.1 system is identical to the 6.1 system except the 7.1 setup splits the rear speaker into a rear left speaker and a rear right speaker. Most DVDs are not compatible with the 7.1 setup. However, many Blu-ray Discs are compatible with the newer setup. With the 7.1 system, you can now experience "loseless" sound. Loseless sound is identical to the original studio recording (as opposed to the compressed audio available in most other DVDs).