There is no difference between the terms beak and bill, although beak is more often used when referring to hooked bills. But for a few exceptions like parrots, the beak is the only tool a bird has and is used in finding and capturing and manipulating animal prey or plant food, preening, courtship, defense, building nests, communicating, and feeding young.
The bill has two parts: the bony skeleton of the jaws and the fleshy covering which is similar in composition to our fingernails. Birds are constantly wearing it down, so, like our fingernails, it grows. Sometimes captive birds have to have their bills trimmed as they don’t wear them down as they do in the wild. The unusual and spectacular Black Skimmer hunts by flying across the surface of the ocean with its lower bill in the sea.
Birds, like us humans, cannot move their upper jaw; the lower jaw does all the work as it alone has a large abductor muscle. Unlike us, the upper jaw of birds can move a bit upward thanks to a special lever mechanism between the two jaws. Orioles, meadowlarks, and other blackbirds have large adductor muscles so their jaws can pierce the ground or a piece of fruit and open to expose the prey or flesh. Shorebirds like avocets can probe an inch or several into the ground and open just the tips of their bills with this mechanism.
Finches use their bill to manipulate and open seeds and some have an astonishing strong bill, such as the grosbeaks or European Hawfinch that can crack open cherry and olive pits. I always used to dread handling Northern Cardinals as they frequently bit me hard enough to draw blood.
Parrots and toucans don’t have bills with much crushing power; instead they open their fruits and nuts by rolling and slicing them like two pairs of scissors. They can cause serious injury to a handler. The bills of eagles and large hawks have sawing, crushing, and ripping power, but the few eagles I have handled were much more interested in putting their sharp talons into me.