There is Dolby Digital (DD) and DTS. They are two companies that present audio in two different ways.
Most DVD's use Dolby sound processing, but sometimes the menu gives you a choice of either format. Some claim that DTS sounds better due to less compression, but who knows! All modern prepro's and receivers can process both formats. I think 5.1 (fronts, centre, subwoofer, and two rear surround speakers) is plenty for most, at least it is for me! Depends on your room as well.