Shag (Phalacrocorax aristotelis)
From a distance they are much harder to separate, the Shag being a bit smaller and having darker colouring all over.
However, the best guide, and a pretty reliable one too, is to say Cormorants like sandy areas off shore or inland seas, rivers and large lakes whereas the Shag is very much a bird of the cliffs and open sea.
If you see a bird diving in sandy Swanage Bay, for example, it will be a Cormorant, but round Peverell Point and head along the Purbecks coast and they will almost certainly then be Shag. Take care, though, whilst a good guide it does not always work! Shag can be seen in Poole Harbour sometimes, especially in the winter months.