Understanding Lenses - from Wide to Telephoto

Zoom Lenses are many lenses in one
Many people new to photography have only used zoom lenses and consequently have trouble understanding just what a wide angle or telephoto lens is. Read on to get an insight!
Pre zoom lenses, one bought a camera with a standard lens. This lens on a 35mm camera was a 50mm prime lens and it gave you the same view as your eye sees.
When you wanted a wider view than your eye sees, you bought a wide angled lens
When you wanted to magnify your image and bring your subject closer to you, you bought a telephoto lens

Sensor Size
So what focal length is a 'standard lens' on your zoom lens?
Firstly, you need to know what size your sensor is. Eg. is it a full frame (same as a 35mm negative), a crop sensor (most DSLR cameras) or a four thirds sensor eg Olympus, some Panasonic and Sony hybrid cameras. All quoted focal length numbers relate back to a 35mm negative - hence calling it 'full frame

Standard Lenses
So... a standard focal length (as your eye sees) is:
50mm on a full frame
37mm on a crop sensor (APSC)
25mm on a four thirds system

Wide Angle
Any numbers below those listed above means you are using a wide angled lens. Using this lens allows you to fit much more of what you are seeing into your picture. Your subject will look smaller and further away, and at very wide angles, objects at the edges will bend inwards. To use this well, think about putting objects close to the camera to create a more interesting picture.

Telephoto
Any of the numbers above the standard lens gives you a telephoto effect. Subjects will appear closer and magnified. Objects in the distance will appear compressed. This is great when you can't physically get close to your subject. It is also good for portraits as it gives a more acurate face shape than wide angle. It also allows you to keep a good distance from your subject so you don't intimidate them or scare them away.