Binoculars are formed by objectives, oculars and prisms. The main advantage is that having a tube for each eye allows the observation in three dimensions, having a sensation of depth to perform the calculation of distances.
The focus is that point where the rays that come from far away converge. Then, the focus of the objective is located at the same point of the ocular. Doing this in that way, the image comes out much bigger than we would see if we only did it with our eyes directly. The result is a much larger image.
The prisms are necessary to put the right image. If they were not there, we would see inverted images. There are the prisms of Porro, which are those drawn in the photo, and the roof prisms, which are those that are mounted in binoculars that are straight, thinner.
There are different types of binoculars:
a) Theater binoculars: they usually have two to three increments. They are often cheap because they do not need any prism-like system to reverse the image, since the ocular is a negative lens.
b) Binoculars of general use: the usual increase is seven to twelve. They need a system with Porro or ceiling prisms to reverse the image. For this reason, weigh more than the previous type.
For larger increases, they need a tripod to stabilize the image