For me, dark elves are a variant of elves, while drows are an entirely different but related race.
Generally I imagine drows as a race without or with a much weaker connection to nature, with gray or black skin as opposed to dark elves' dark brown or purplish, much rougher in appearance and character because of different upbringing.
All of this is gonna heavily depend on the setting though, mainly whether the setting has both or not. Most will have only one, I assume, then there's a lot more freedom.