What is the difference between CL2 and CL3?

CL stands for CAS (Column Access Strobe) Latency, which is the amount of time that it takes to retrieve data from the module. CL2 and CL3 refer to the amount of clock cycles that it takes before the initial stream of data is sent. CL2 modules wait two clock cycles before sending data. CL3 modules wait three clock cycles before sending data. CL2 modules are faster since they only wait two clock cycles and therefore are usually more expensive. Some systems may require either CL2 or CL3 memory.