Hwrm, in uni now...so
*erase* blackboard. Enough of that guru or self righteous or senate crap.
Let me give it a try..*rub hands*
Center weighted metering on the whole concentrates a meter sensitivity at the center of the image, usually designated by a small circle in the SLRs. Unlike spot metering, consideration is also given to the outer area of the images, and the metering areas are relatively large. They are often given as ratios, such as 60/40 and 80/20, which means 60% of the meter reading is concentrated from the metering circle and 40% from the overall corners of the image.
This is the predominant early metering before matrix metering in the early SLRs. The most common one too.
This a relatively simple meter pattern to use, ideal for hand held photography. All you have to do is search for a large part of the photograph which is of an even tone, meter that, and meter another part of the photograph with a different tone, and count the number of stops between the 2 tones (usually the darkest and brightest). From there you can either select the middle value (the easiest but not always 100% reliable way to get what you want) or apply the Zone system in the most elementry form.
Spot metering on the other hand concentrates the meters sensitivity in a tiny area, or spot, usually 3% of the picture area. What it gives is precise measurement of any small or multiple area of the photography, enabling you to get a very clear idea on what the exposure field of the photograph is going to be like. You can determine whether the scene effectively fits into the dynamic range of your film or sensor (Read this up on the net.) compared to the center weighted system. The Zone system was developed with this metering in mind (Very commonly used in LF cameras), and offers unlimited amount of control at the expense of time. No snap and go like Henri Cartier Bresson with this mode =).
All in all, different metering modes gives you essentially a proper exposed photograph if you learn how to use it, it's just some of them are more suitable for certain situations than others. Like the center weighted metering is good enough on a cloudy day (and alot quicker too), but on a contrasty high noon situation, a spot meter would tell you enough of the scene to allow you to choose the best exposure value to take that photograph.
Depends how you use it.
(To the thread starter: Read about the zone system and you'd understand alot more what is exposure...you could skip the technicalities but at least have a know how to apply it.)
(And as for all the ppl here...all this information is available on the net or in books or in libraries. Don't plop into clubsnap and expect ppl to cater to you for spoon feeding session. Research!! You have all the worlds expert's opinion on your fingertips or paper for over a 100 years of photography, make use of that. Only when you don't understand, pop a question in here..)