there are so many different types really
your best bet for getting an idea might be to go to the classified sections and look at all of the different leagues advertised and then come back here and maybe ask specific questions about things you might not understand
i am in 5 leagues and all of them are different some have minor leagues some dont
some minor leagues are call up and down some are call up only
1 allows 'prospects' to carry over free regardless if they are in the minors or on the big club
some limited keeper some are full keeper/dynasty
but yeah if you go to the classified sections you can get an idea for the rules for some of the leagues
maybe find ones that list the rules and the rosters and give a try at picking the keepers for the team and posting them for others to discuss thus gaining an understanding of who should be kept under what settings etc
your question is very broad and without knowing specifically what aspects you are interested in learning about pretty tough to answer