The diagrams above represent the way in which these three main metering patterns work. From left to right:
1. Multi-pattern metering (the camera's default) breaks the scene down into segments, measures the light in each and then makes an 'intelligent' decision about the type of scene it's looking and the best exposure to reproduce it correctly. Multi-pattern metering is more likely to be right, more of the time, than any other system, but its response isn't easy to predict.
2. Centre-weighted metering is much cruder. It measures the overall light value, like older metering systems, but places more weight on the centre of the image, where the subject (it's assumed) is most likely to be. Centre-weighted metering is more likely to get it wrong, but, if you're an experienced photographer, it's predictable and easier to correct than multi-pattern metering.
3. Spot metering takes a reading from a small area in the middle of the picture. It's useful when there's a big difference in brightness between the subject and the background or between one part of the scene and another. You use it when you want to base the exposure on just a single area. It's easy to get it wrong, though, and not quite meter the area you meant to, or choose an object that's intrinsically light or dark. Spot metering magnifies your mistakes alarmingly!
