1) It's possible to steal land, tech and money, in proportions that are almost entirely dependant on your own current tech level. There is a base value though (I think) which was, back in the years 1 and 2 before the great sausage war, 20 land, 5 tech and the money is capped at 1 000 000.
2) You attack, you win ground battles, you get tech among other things. (see previous)
3)At lower NS, yes, it is worth it. at higher levels you should start buying your tech since A ) you will not find any targets B ) In the long run it's cheaper to buy the tech instead of possibly losing infra to an active raid victim. Although, it's still fun to raid even though you don't really gain anything from it
4) no, not really. Unless you do something extremely stupid and raid an alliance member.