CL refers to the CAS latency of the memory. The lower the better. That's obviously the simplest definition. If you're looking for something with a bit more depth, you can check this out:
http://www.mushkin.com/mushkin/pop-up/latencies.htm' ">http://www.mushkin.com/mushkin/pop-up/latencies.htm
http://www.lostcircuits.com/memory/dimms3.shtml' ">http://www.lostcircuits.com/memory/dimms3.shtml
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Intel Core i7 860
ASUS P7P55D-E Pro
Corsair HX650W
Corsair XMS DDR3-1333 (4GB @ 8-8-8-24)
Sapphire Radeon HD 6870
Western Digital VelociRaptor 300GB
Western Digital Caviar Black 1TB