I have a stupid question about cloth diapering.

What my question is, is how does a fleece cover keep the wetness in the diaper, and not on the outside? My mom clothdiapered, but she used these plastic cover things over the diaper (which don't look very comfortable). They were like the ones that I used when potty training Randy. I used 5-ply cotton training pants with the plastic cover over them. With Jenna I bought like AIO training pants. But even they have a plastic cover on them. SO my question is how do the covers that aren't plastic (I REALLY don't want to do the plastic covers) keep everything outside dry? Can you use them with more than one diaper, or do the covers (like fleece, or wool which I probably won't use because of lanolizing) need to be changed everytime with the diaper? What do you use under them? Prefolds or fitted or does it not matter? Sorry so many questions, I need a book like cloth diapering for dummies. lol