|Three exposures of one raven in flight (internet photo)|
Cornell University has audio at http://www.allaboutbirds.org/guide/Common_Raven/sounds.
Beak to tail-tip, ravens are 50-60% larger than crows, and weigh twice as much. The head and neck is larger in proportion to the rest of the body. In flight, a crow's tail is straight across the back end, while a raven's widens, then narrows. Both can walk or hop (many smaller birds only hop).
As to why we sometimes see small birds in flight chasing and harassing much larger birds, that is nest defense. The smaller birds are agile enough to fly above/behind the larger bird, then dive in for a peck or two. On a different scale, in Alaska, ravens have been known to harass bald eagles in the air, and on the ground, when feeding at the same carcass (left behind by wolves or polar bear), to sneak up and pull an eagle's tail feather.