The books that Thor recommends are great (I own them), but maybe not for the novice. If you want a good solid intro for the questions you have, I would recommend going to Amazon books and picking up Sydney Painter’s Mediaeval Society and The Rise of the Feudal Monarchies. They are about 100 pages each, with sections broken into readable chunks and the font size is easy on the eyes. You can pick up used copies for a buck or two each.
In fact I’d recommend these two books for anyone who wants a good mental picture of medieval times for RPG gaming. They are a little dated (1960’s?) and there’s a lot of good books out there, but they are a good place to start. I always find myself rereading them every few years.