This might be a good place to start:
http://en.wikipedia.org/wiki/Categor...ll_terminology
To answer your specific question:
A "closer" is a pitcher who is used at the end (usually only the last inning) of a game when the pitching team is leading by a small (1-3 run) margin.
A "middle reliever" is basically any other pitcher who is not a starting pitcher or the closer - the exception is the "set-up man" who usually works the inning
before the closer.
If you want to get the hang of it a bit, you can always download the
free version of OOTP5, so you can kind of get familiar with some of the terminology and financial aspects of the game until you can afford the quantum leap to 2007.