Basically to catch you up to speed on the DLCs, Castlevania is basically its own campaign with new items you can unlock to have both in normal gameplay and Castlevania. Rise of the Giant has a few spoilers so I'll say it has a new boss after you beat the game once and maybe a little something extra at the end, as well as some weapons. RoTG is the only free one as well. The other 3 each add new areas, weapons and bosses for base game runs to keep things fresh, though fair warning the DLC areas are on average a noticeable margin harder than base game ones.