Pretty well covered already.
Keepers and Dynasty hold players year over year, without any penalty. You can keep them until they retire. If there's a limit, that usually means it is either (A) a contract league, (B) a salary cap league, or © an accelerated cost league (like when you keep a player year over year but each year his "price" goes up - like a 4th round pick year 1, 3rd year 2, etc.
General thoughts are that if you keep fewer players than you start on a weekly basis, that's a Keeper league. If your league keeps enough players such that only your rookies can really fit on your roster next year, that's a Dynasty League. There are subtle grey areas between those two, but IMHO once you have enough players kept to fill out your starters, well, you're teetering on Dynasty status.
(Example - 20 man roster, start 1-2-3-1-1-1, Keeping 9 is borderline Dynasty. If you have a 4 round rookie draft and keep 16, you're Dynasty. Keep 10-15, you're in the grey area in between).