The series is the main thing, for example Core 2 Duo, Pentium 4, Athlon X2.
The speed in GHz is important, and so is the FSB speed - the higher the better.
Finally, the cache is measured in Mb (the more the better) and the number of cores (1 core isn't so common nowadays, dual core seems to be the main standard, and quad core is becoming the standard for high end PCs). If you're a heavy multitasker you should go for more cores.
Does that help you out?