I'll get to how this effects sound quality a little later on. The loop information is very important for use as an instrument. If a note is held longer than the sample length, the player needs to loop back and play part of the sample over again to keep the sound going for the requested amount of time. The loop type tells it whether it is a normal, bi-directional, or reversed loop. It can often be tricky to choose good loop points, I'll discuss this further down as well.
Every sample is associated with a Region.
Each region is associated with one sample, and tells the player almost everything else it needs to know about using the sample in an instrument. First, the player needs to know what range of notes is associated with this particular sample. The other important information stored in the region is all of the Envelope settings, including chorus, reverb, and panning overrides. Adding a good envelope to a patch can usually greatly improve how it sounds, by making it sound more realistic by setting volume ramps for Attack, Decay, Sustain, and Release regions of the sample, plus adding a base level of reverb. Each Layer is just a collection of regions.