BigCub: I believe d2, NAIA, and Canada allow players who have become professionals can play college ball, but for every year played as a pro, at least a year of eligibility evaporates (and there are other rules which may apply) (except Canada [i believe]. (My info is pretty dated, however.)
King: all the information you seek is found with a Goggle search. It would be more productive to bring somewhat of an understanding how the system works and then ask questions, as opposed to dropping a vague and ambiguous question to an anonymous baseball forum. There is a lot of material on the MLB , Baseball America, Wikipedia, and Perfect Game websites, amongst other places - which will build a foundation upon which you can then ask questions to fill in blanks or which would otherwise be specific and not general.
There is so much to learn, and we all started from scratch; but, the info is out there and easily found.
(BTW, there was a kid who was drafted and signed out of HS who went to Stanford before he began playing; his career was non-existent. In pre-draft discussions, we tried to get teams to agree to allow S to attend at least his first semester on our nickel (school has a rule that once you matriculate and leave in good standing, you can return to finish). We were patted on the head and politely told that wasn't the way things work.)