If you own a Digital SLR or Micro System Camera, or are thinking of buying one, it's likely that at least part of the attraction is the ability to attach a variety of different lenses. So let's assume you've got a bit of extra cash burning a hole in your pocket; which is the right one for your needs?
Lenses come in a huge variety of focal lengths from extreme wideangle to super telephoto. Some of them are zooms, while others are fixed. Some use special types of glass to enhance their performance, while others employ built-in motors to speed up their focusing times. A few feature image-stabilisation (IS) devices to reduce the risk of camera shake. Bear in mind that you'd only need this if you use Canon or Nikon, as most other brands have IS built into the camera.
Often manufacturers offer more than one version of the same focal range. It may be that they have both a full-frame model for cameras with 35mm sized sensors, and another for the smaller APS-C sized sensors, which are often more compact because the image circle it produces is smaller. Or there may be a premium version with a wider maximum aperture. This offers benefits including a brighter viewfinder, faster focusing, the ability to use faster shutter speeds or shoot handheld in lower light, and the ability to achieve shallower depth of field. The flip side is that these faster lenses are much bigger and heavier than their slower counterparts.