Good AI summary above, but a few more notes:
No transfers, at least not the way you know them. New players enter the league via the draft (with some exceptions). Once a player signs a contract, the team owns that contract until it is traded or expired. (Traded players make the same money after a trade, no new contract like with a transfer). So trades are most like transfers, but don't usually occur for money; usually it's a player swap or includes draft picks to balance the trade. Sometimes a bad team will "eat" a bad contract, and this is important because the salary cap is an important part of the league structure. No team can exceed the salary cap (or go below the salary floor), so sometimes a good team can't keep all of its players because it can't afford them (against the cap).
An example of the last bit is the Dallas Stars need to make a decision on two good young players who are restricted free agents (which means they have partial bosman rights, but only if their team does not agree on a new contract): Jason Robertson & Mavrik Bourque make $7.75M and $950K against the current salary cap but are expected to get paid ~$12-14M and $3-4M on new contracts. The Stars have a few expiring contracts and the new cap is higher next year, but they will either have to trade another player or let players walk to open up cap room for these contracts, or they'll have to trade one or both of these guys away if they can't agree on a contract. In the trade they'd likely get a lesser player or a future draft pick.
Great teams have great players often because they are smart in the way they manage draft picks and contracts.