I am often asked what the difference is between a cormorant and a shag (Phalacrocorax aristotelis) and the simple answer is that there are many differences when you see them close up and side by side! From a distance, however, 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 inland waters (rivers and large lakes) and sandy areas off shore whereas the shag is very much a bird of the cliffs and open sea; indeed I should really have included the shag in the seabird series..
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 and cormorants can be found on open sea.
Finally, a bit of nonsense I learned at primary school: the lesser cormorant or shag lays its eggs inside a paper bag. At night bears come, eating buns, and steal the bags to hold the crumbs.