CPU speed is how fast the actual processor clicks, or the amount of bits it can push through per second.
The System clock is equivalent to the bus speed, which used to be either 100Mhz or 133Mhz. Nowadays the System clock might be 266, 400, 533, 800....
The kicker behind these new numbers is that some are "double pumped" or "quadruple pumped" slower buses. The 266 on the T-birds was a 133Mhz bus "double pumped" or using dual data rate transmission. Since the bus supported a bit coming onto the bus while another moved off, it simulates a 266Mhz bus. The same held true for the celeron's when they first introduced the 400Mhz bus. I haven't followed processor architecture since about 1.8Ghz, so true bus speeds might have changed. The Athlon64 series and the P4 processors with HyperThreading might be totally different from when I actually had to know this stuff, but this should give you the basic idea.