I live in southern New Hampshire, and there are many large black birds. They often attack thermalling hawks. Are they crows, or ravens? How can I tell them apart?
It’s a crow! It’s a raven! NO! It’s a Flying Monk!
My mom the vet told me that ravens are bigger than crows, and that that’s the main difference. She’s small animal though, so you might want a second opinion.
It’s a Super Gnat!
Corvus corax, the Common Raven, can be characterized by a comparatively larger size, louder and deeper “caw” and a more rounded tail than Corvus brachyrhynchos, the American Crow.
There’s a much better answer [url=http://birds.cornell.edu/crows/crowfaq.htm#raven]here*.
Could I possibly mess up that link any more? I hate posting on Mozilla. 
I can’t use the neat and tidy shortcuts…
Ravens are larger. Trust me, we have lots of big black mutha here in Alaska.
Hey, sailor, you’re in fine form this evening!
I’ve been doing some surfing, and it seems that they’re American Crows. Ravens apparently have some wacky hairdo.
A crow is not like a writing desk. 
A raven is like a crow, only bigger, if that’s possible. (I didn’t think it was, until the first time I saw a raven. Damn, them things is HUGE!) Also, I think crows have a smoother bill, whereas a raven has a little bump up near the top. Or maybe that’s just part of the wacky hairdo!
Heh. I’ve always liked that riddle. 
Crows and ravens (and magpies and jays) are all members of the Corvidae family. There are nearly 120 species of corvids in the world (or more if you follow Sibley and Monroe’s classification). Corvids are characterized by bristles that cover the nostrils (except for the piñon jay), and the sexes look alike. Corvids are in the Order Passeriformes, which means they’re songbirds! You normally don’t hear crow songs, but it is a soft, warbling sound that belies their passerine origins.
As far as telling the difference between crows and ravens, the answers are basically given above: Ravens are much larger, have relatively more massive bills, have a wacky 'do (“horns” or feathers on the tops of their heads that can be raised), a wedged shaped tail, and a rougher call. But these characteristics are not always easy to see. The horns are not always raised, and tail differences might not be evident. If the bird is in the air, you might not be able to tell the size difference unless you have something of a known size to compare it to. But here are a few more clues: Ravens are more likely to soar. They are found away from human habitat while crows are attracted to how we live. You are likely to see ravens in montane and desert regions. Given this, I’ll listen for raven calls: a deep krruk krruk verses a caw (brachyrhynchos) or an uh-oh (Fish Crow, C. ossifragus).
They are different species of the genus corvus:
here is a page with some image links.
My friend told me that one species, I think crows, have black beaks, while the other species, I think ravens, have yellow beaks.
He says he knows this because they line up on the wire outside of his office window. Apparently he’s been doing a lot of staring.
However, the images on the links given here do not seem to bear him out. Perhaps he is hallucinating due to overwork.
Ravens have black beaks and if you see them (the best place to be sure to see them is the Tower of London) you will see they are appreciably bigger than crows.
The only black(ish) birds with yellow beaks I can think of are starlings, that also have the habit of sitting on wires.
Starlings are very much smaller than most corvids though; the way it works here in the UK (where ‘crow’. ‘raven’, etc) may refer to different species than in the US is:
“Oh look at that crow” = Crow
“Oh look at all those crows” = Rooks
“Bloody hell! that’s a big crow!” = Raven
Ravens don’t have yellow beaks - Maybe your friend is thinking of the female blackbird - she has a yellow beak. At least here in Ireland… Ravens have black beaks - defo.
Again though, blackbirds are very small compared to crows and other common corvids.
AFAIK the only place crows have yellow beaks is when they appear in cartoons or as stuffed toys.