Keeper - you can keep a portion of your roster from year to year. generally 1-4 players. There are different rules as to what it cost to keep a player, some the round the kept player was drafted in the year before, some the round or 2 prior to when they were drafted.
Dynasty - keep your entire roster form year to year. The way to improve your team is through a rookie draft each year and trades. There are also waivers but generally it is slim pickings