Focal length is the distance between the image sensor in a camera and the centre of the lens, and it determines the angle of view. The angle of view depicts how much of the scene you want to capture. The focal length on high end cameras states 50mm as a normal angle of view, 10-18mm as a wide angle view and 30mm+ as a small angle view.
So if you wanted to take a portrait photo, you would use a regular lens, for a landscape or interior photo you would use a wide angle lens, and to capture wildlife or a sports game, you would use a telephoto lens (name of small angle view lens.
On most cameras though, the standard range is 18-55mm, 18mm being wide angle, 35mm normal, and 55mm telephoto.
In this set of photos, my model and I stayed in the same place and I changed the focal length for each photo. The photo in the middle was taken with a normal lens (35mm) the left was taken with 18mm, and the right with a telephoto lens. You can see the difference focal length makes to the angle of view.
Focal length can also alter perspective. In the above photos, my model stood in the same place while I moved backwards and forwards so I could keep his knees and rest of his body in each picture. Doing this clearly shows the illusions of different perspectives. In the photo on the left, taken with an 18mm lens, the background (the wall) looks further away from the foreground. The opposite effect takes place in the photo on the right, which is taken with 55mm. The picture in the middle is taken with a normal lens and therefore has the correct perspective.
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