Stats act like... Well, I guess the best way to put it in terms would be like modifiers. In layman's terms, you do things related to them better the higher they are. So... You hit harder with 200 Str than you would at 100 (though this is merely a relative term; stats don't mean everything in this world).

Magic affinity is... Well, pretty similar to what it sounds like. It basically shows what magic a character is more oriented towards using (assuming they are ever able to get any). There's earth, water, fire, wind, holy, unholy, neutral and almighty (the last one usually being reserved for gods and the like).