This is a rook:
Note the whitish beak and featherless face.
Now take a look at a crow: This is a carrion crow. Black beak, face fully feathered.
They're very, very similar. Genetically very close, feeding habits and habitat very similar too.
So how do you tell them apart?
Well, it's easy. If you see a lot of them together then they're rooks. If you see just one or two at a time, then they're crows. It's true, it's a simple as that.
You see, crows are independant and like to do their own thing. Rooks, on the other hand, have to do everything en-masse and become terrified of being left on their own. Worse still, rooks become positively panic-stricken when a particularly bold member of the rookery decides to go off and do his own thing.
If a very naughty rook decides that he wants out and maybe even start up his own rookery, he will be attacked and dive-bombed by the other members until he comes back into line. Only the most courageous and unusual rook will overcome his fear of being alone and stand firm whilst he does his own thing.
Englishman.