All right, OOTP. It's a giant simulation of everything. You cannot actually go up there and swing the bat, arcade-style, like you could in Earl Weaver Baseball (previous titleholder of Best Baseball Game ever), but it's got everything else. You can be the manager and have the computer be your GM. You can be the GM and let a "computer manager" manage the team. (To the point where you can hire and fire managers based on different criteria.) You can be both the GM and the field manager, which is what I liked to do, although I let the computer simulate most of the games so it didn't take six months to play a season. I always played as the GM, set up parameters for my computer manager as to how often I wanted him to issue IBBs, bunt, play the infield in or out, etc etc etc, and then step in myself to manage big meaningful series or the playoffs. You can really control as much or little of it as you like. Trades, amateur draft, minor leagues, player development -- it's all there. I have always played by myself, but my understanding is that you can set up a league with one human-controlled team, a few of them, or all of them. Whatever you want.
You can start with real teams/players and go from there if you want. Players age and retire; the computer generates new guys to replace them for the draft. Maybe I'm weird, but I've always preferred to go with the tabula rasa and play with completely fictional teams and players. It seems purer that way.
I used to spend a ton of time on it during the offseason, and then it was one of the things that got elbowed out of the way for the kid(s). I still buy the new version every year, mostly to support the product; I update my team and league; and then I rarely play it. Great game though.