These two birds at first glance are very similar , they are black long necked often described as reptilian in their looks and found near or on water.
Cormorants can be found inland on fresh water and at the coast whereas Shags are only found on the coast.
This bird with its wings out in the classic pose is a Cormorant on top of a tree at Rushden lakes right in the centre of the country . 
Cormorants are a larger bird than a shag , their feathers are a black/brown colour. Their face has no feathers around the green eye. They also have a distinctive white patch if feathers on their thigh area and also under the chin. They also sit low on the water surface. In flight they resemble a pterodactyl !
The Shag has a smaller head and a longer more snake like head.
It has a feathered face and an emerald eye. The plumage is black/ green and glossy. In the breeding season it has a tufted crest.
Cormorants and young on the rocky outcrops in Brittany,
A cormorant sitting low in the water ( this is one of the identifying features)
From a distance it does seem quite difficult to see differences . Practice needed, an excuse to go Back to the wonderful Brittany coast.
More information can be found on rspb.org.uk
NATURE TABLE EXPLORER 