Sorry for the really newbie question re: freewheel/cassette

tavyabe

I know this is probably as amature as questions get but despite reading sheldon brown I can't tell the difference. Can someone let me knwo if this is a casssette or freewheel. Also my chain seems to be slipping, can you tell from these pics if it needs replacing or is it likely to be something else causing it to slip?

Many thanks

nicklouse

cassette and worn.