You mean the difference between evaluative/matrix, centre-weighted and spot?
Evaluative is the one I always use. It takes lots of reading from all over the screen, dozens of them, measures and compares them, references them against a massive database of similar image characteristics, and presents you with what it reckons to be an optimum exposure. It works very well, isn't often fooled, and is dead easy
Centre-weighted measures and averages the whole scene, but with an emphasis towards central subjects.
Spot metering takes a reading only from the small circle in the centre of the viewfinder. TBH I think the circle is too big to be useful as a true spot meter but, unless you know exactly what you're doing, use it at your peril.
IMHO, with digital there is no need to stray from evaluative. It will always get you close, if not perfect. Then chimp the LCD, have the 'blinkies' over-exposure warning enabled and learn to read the histogram. Adjust with +/- compensation on Av, Tv or P. The manual mode merely locks the exposure settings, it doesn't make the exposure any better, but use that if you prefer.
There is only ever one optimum exposure setting. The various metering modes help you calculate what that might be - obviously essential with film. With digital, the LCD image and the associated aids show you
exactly what you
actually have got.
