That took a search way back in the archive to find the first link that made me start to understand metering...
It's called:
Understanding Metering
It may not be the most in-depth site out there, but it made a lot of sense to me when I was just learning because it shows you what difference it makes with pictures and without getting overly complicated.
Some other useful resources are:
Metering Modes (Thread)
Selecting the Metering Method (Thread)
Understanding Camera Metering and Exposure (Website with lots of useful tutorials on other things too)
Hope that helps!
