Binoculars use prisms that redirect incoming light in different directions allowing it to travel a long distance in a small amount of space. A long focal length can be used in this way. Without prisms, the sides of binoculars would be as long as telescopes.
Most binoculars use either a porro prism design or a roof mounted design.
This gives them a sleek look and makes them smaller and lighter-weight because large offset barells are not needed. For most people, they are easier to handle and focus..
They are also usually more rugged. With porro prisms the eyepieces moves in and out. In roof prisms the eyepieces do not move.
Typically, roof prism binoculars have been more expensive because more labor was used to make them. With updated construction methods however, the price of production has greatly been reduced. Now roof prisms with excellent quality can be had for reasonable prices.