The first thing to bear in mind is that 50mm is considered a "normal" focal length, as you'll see pretty much the same through the viewfinder as you would with the naked eye, if you are using a full frame camera. This is a good starting point to bear in mind, as this will help give you an idea of how the rest of the focal lengths compare.
If you have a cropped frame camera, please note that the focal lengths as described below won't have the same field of view on your camera. (If you are unsure, you probably have a cropped frame, most entry or mid level range cameras are cropped frame) It's all to do with the fact that you have a smaller sensor, which makes the lens perform differently. What you need to do is multiply the lens focal length by your cameras crop factor to get the "equivalent" look through your viewfinder. Different cameras have different crop factors, but Canon's is 1.6, so I'll use this to give you some examples. Let's start with the "standard" 50mm. On a cropped body, instead of the scene through the viewfinder being the same as the naked eye, it will look a lot closer - more like a 85mm lens would perform on a full frame, and therefore falls into the portrait range on a crop. How do I know this? I take 50mm and multiply it by 1.6 and get 85mm. And an 85mm lens on a cropped body will perform more like an 135mm lens (85mm x 1.6)- so in the telephoto category. If you were looking to get a lens that was roughly equivalent to the naked eye on a cropped frame, you would need a 35mm. (35 x 1.6 = 50)
So, if you bear in mind that the focal lengths are described as how they perform on a full frame, and make the necessary adjustments for the cropped frame, you'll get a good idea of what each focal length does.
Now onto the focal lengths!
Under 20mm - Ultra Wide Angle
Wide Angle essentially allows you to capture more of the scene than you would with the naked eye, essentially how "zoomed" in the image in your viewfinder looks. Ultra wide angles are used a lot in landscape photography as they can fit a lot more of the scene into the viewfinder than can be seen with the naked eye. Never, ever use for portrait work!
20-35mm - Wide Angle
Wide angle lenses are the same - they squeeze more into the frame than can be seen with the naked eye. Don't be tempted to use this range for portraits as this will lead to distortion in the face - they can make the nose look bigger and wider, and the face narrower than it actually is. This distortion can sometimes look cute and whimsical on a child, but you certainly wouldn't want that for every shot. 35mm is great for full body shots, or group shots when you don't want to be too far away from your subject.
35 - 70mm - Standard Angle
This is considered a "normal" range in photography - you are seeing (more or less) as you see with the naked eye (remember 50mm is considered standard) At the 35mm end it is great for body shots or groups shots. A 50mm is a good all-rounder and a camera bag staple.
70 - 105mm - Portrait
The 70 -135 mm focal length is generally considered the best for portraits. This is because there is no distortion to the facial features at this focal length, and generally you are far enough away from your subject to get some pleasing bokeh, but close enough that you can interact with them. 85mm is probably considered a staple portrait lens.
105 + - Telephoto
Longer focal lengths are called telephoto and can also be used for portraits - 135mm is considered a great focal length. It tends to slim the subject slightly (woo hoo!) making it very pleasing! They can give you very buttery blurred backgrounds, and you are a little bit farther back from your subject which is great for candid shots outdoors. Moving up the focal lengths, longer telephotos are used for sports or wildlife where you are a distance away from your subject. You may need focal lengths of 200 or 300mm or even 600mm, depending on what you are shooting.
Remember too that you can have different focal lengths on a zoom lens - an example of a telephoto zoom would be 70-200mm , and a mid-range zoom around 28-75mm .
Next week, providing I can get my husband Adam to agree to it, I'll take a portrait shot with lenses from the wide angle, normal, portrait and telephoto ranges, so you can see the difference the lens makes to the final shot - from how much you can get in the frame, to how the different lenses changes the "look" of your final image. (All together now, say "Please, Adam?!" so he'll do it for me....)
Other Posts You Might Like"
What do the Lens Numbers Mean?
What is the Nifty Fifty Lens?
Zooms Vs Primes