We’ve all had the experience before. You’re trying to take a photograph of some friends, but you just can’t fit everybody into the picture. You step backwards further, but there’s a wall or cliff or something, and it just isn’t going to work, so you tell your friends to squeeze in closer. Or you see a bird flying by in the distance, you grab your camera, and you end up with a big photograph of sky with a disappointingly tiny little dot in the middle of the frame.

In each case the field of view provided by the lens isn’t appropriate for your subject matter. In the first instance your lens is not “wide” enough to take in the whole scene, and in the second your lens isn’t “long” enough. There are three rough categories of lenses when it comes to how much of a scene they can take in, and the field of view of each type is defined by an optical property known as the focal length of the lens (a property explained later in this document).

A so-called normal lens roughly approximates the perspective, though not the area of, a scene seen by one human eye. By convention a normal lens on a 35mm film camera (and thus a full-frame EOS digital SLR) has a focal length of 50mm or so. Think of normal lenses as being good for taking pictures in close, but not intimate, proximity to a subject, like a waist-up picture of a person in an ordinary room.
A wide-angle lens can take in a large area of a scene. This has two common applications - first, it means you can take in sweeping panoramic landscape scenes, and second, you can take in large areas of an ordinary room. If you want to take a photo of a group of friends at a dinner party you’ll need a wide angle lens unless you can back up far enough to get everyone in. On a 35mm film camera a wide angle lens would have a focal length of, say, 35mm or less.
Looking through a telephoto lens is like using a telescope - it narrows down what can be seen in a scene or makes the subject seem much closer than it really is. A telephoto lens might have a focal length of 70mm or more on a 35mm camera.
Note one area of potential confusion - there are different measurements expressed here in millimetres, but which refer to two different physical properties. In the case of 35mm film cameras, we’re talking about the width of the film material. But in the case of, say, an 80mm lens, we’re talking about its focal length. You can thus put a 28mm lens on a 35mm camera - the measurements are about different things.

These are just broad categories, of course, and there are big variations in each one. You can get an inexpensive 28mm lens, for example, which is only modestly wide. Or you can get a crazy expensive 14mm lens which can take in a huge area of a scene - perfect for shots of ultra-dramatic skies. Similarly you could put an 85mm lens on your camera for portrait photography, or you could sell your car and buy a huge 600mm lens that requires a large suitcase for transportation but which lets you take a closeup of a bird’s face from a huge distance away.