Slowly, but surely, the worth of the fixed focal length lens (or prime lens) to the non-professional photographer is being eroded by the increasing perfection of zoom lens technology. The days when you packed your camera bag with trusty 28mm, 50mm, 85mm, 105mm, and even 180mm lenses are rapidly disappearing. Why bother when you can simply reach for something like Nikon's all-purpose Nikkor 18-200mm DX VR zoom lens and know that you have both flexibility and optical speed at your fingertips?
The truth is that, while the range of focal lengths may be increasing within a single zoom lens, no all-purpose lens is ever likely to perform at the level of equivalent prime lenses for its full range of focal lengths.
Zooms are engineered with a good deal more glass than is found in a prime lens, and because of this they "swallow" more light than the corresponding lens of the same focal length. Unless you are photographing under very bright light, the required increased shutter duration time of a zoom may make the difference between a good image, and one that is ruined by camera lens wobble.
Even so, there is a lot to be said for carrying one lens, and leaving it attached to your camera, rather than carrying five separate lenses and having to miss photo opportunities that arise while you are changing lenses.
But thinking about fixed focal length lenses can be instructive. When we plunge into photography with a zoom lens, the effortless continuum of focal lengths is apt to blur the distinction between photographs captured at different focal lengths. Historically this just wasn't a problem. Economy demanded that the lenses you purchased were quite distinct, focal length wise, and so an appreciation of their individual characteristics came easy.
50mm
It used to be the case that the kit lens for a 35mm SLR was generally a 50mm focal length lens. The 50mm lens reproduces a field of view that approximates normal vision, with an angle of coverage of around 50 degrees. But this is also partly the reason that the 50mm lens gets only a fraction of the respect it deserves. Because when we look at the scene captured by a 50mm image, we are not going to see something that we could not have seen with our own eyes. This means there are no optical effects to draw us into the image, so the subject material must be strong enough to command our attention.
Despite the fact that the 50mm kit lens has been replaced with an optical zoom lens (usually in the range of 28-80mm these days) the 50mm lens has a lot going for it. For one thing it's generally very fast. My 50mm kit lens (picked up decades ago) is an f/1.4 Nikkor. By dialing up to the largest aperture you can get away with not using a flash when taking indoor photographs in natural lighting. This means that your shots will look more natural, and have a depth to them that washed-out images captured with flash simply cannot match. You'll also be able to shoot fast. The equivalent focal length on a zoom might be two, or even three, stops slower.
If you will be using your camera mostly to record family shots, and you will be shooting indoors a good fraction of the time, you might want to take a serious look at picking up a 50mm f/1.8, or even an f/1.4 if you have the money to spend. To read a fine essay on the under-appreciation of the 50mm lens, I recommend The Forgotten Lens by Gary Voth.
28mm
Open up a book on photojournalism and you'll discover a slew of images captured with wide angle lenses like the 28mm prime lens. Wide angle prime lenses have a remarkable pronounced curvature on the front lens element which helps to pull in light from all around the edges of a scene. This has two main consequences.
The first is that a much larger portion of the scene before you is captured. Extremely short focal length primes, like the 8mm, are known as fisheye lenses because the angle of coverage exceeds 180 degrees. Everything in front of the camera ends up in the image!
The second consequence is that cramming more of the scene into the image distorts the image to some degree. Straight lines in the real world, like the horizon and the edges of walls, become curved. Most of the time this is acceptable, because capturing the entire scene is the most important consideration.
Using a wide angle prime like the 28mm becomes necessary when you try to capture images in confined spaces. To shoot all of the board members in the annual meeting while they sit at the conference table, and to get all of the table plus the room around it into focus, requires a wide angle lens. The other situation that calls for a wide angle arises when you find yourself trying to photograph a very large object that won't quite fit into the frame. If you were taking a picture of someone standing at the base of the Eiffel Tower, using a wide angle will help you fill the frame with both the subject and the tower.
85mm to 105mm
I have mentioned how useful the 50mm prime lens is for doing portraiture. However, the lens most often mentioned in connection with portrait shots is the 85mm prime, usually with an f/1.8 maximum aperture, or the even faster f/1.2
The 85mm is sometimes referred to as a short telephoto. Unless your aim is to capture just the upper torso of your subject, or simply their face when closer in, you will need to back off to capture your subject from head to foot. This means that you can often take unobtrusive and spontaneous portrait shots simply by being in the vicinity of your subject as they go about their activities.
But the real strength of the 85mm, or even the 105mm, prime, lies with the ability of the lens to get in tight and emphasize the features of the subject's face. The background in these shots is not the least important, and by opening up to the maximum aperture you will be able to throw the background completely out of focus. Focusing becomes critical in this situation, so concentrate on the subject's eyes which can never be unsharp in acceptable portraiture shots.
180mm
There will always be times when you cannot get close to the scene that you want to photograph. This is where telephoto lenses come in handy. Medium telephoto range like that provided by the 180mm prime is useful for photographing local sports events, where you can approach the edge of the field.
Prime telephotos capture only a small portion of the scene before you, so in order to pull in enough light that you can see the scene properly, the size of the lens has to grow. In other words, your 180mm lens is going to weigh you down a little. For this reason, even if you are photographing at maximum aperture, it is a good idea to use a monopod to steady your shot. Hand holding a telephoto quickly becomes not only a chore, but a great way to lessen the likelihood of a sharp image. This is where the addition of vibration reduction technology to the lens (or image stabilization, as it is also known), can be very advantageous (but extremely costly for telephotos).
Like the short telephotos discussed already, focusing becomes even more critical here. Unless your subject remains relatively still, or your lens is capable of rapid auto focusing, some real amount of practice is required before you can consistently shoot great action shots with a telephoto. On the other hand, the results are worth it.
So to wrap up, I have summarized the characteristics and uses of several different focal length lenses. Try to keep these characteristics in mind if you are shooting with a zoom that covers some, or all, of the focal lengths discussed here. If you think about your shots in terms of these different focal length characteristics you'll be better able to harness them to improve your photography.
To help you select a suitable digital camera to get started with, I have put together an article for you about how to find the right Beginner Digital Camera.
Whether you need a simple point-and-shoot model, or a more complex digital SLR model, you will find the answers, and greatly discounted digital camera offers, at http://www.bestdigitalcameradiscounts.com/
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