CAS latency (also referred to as latency) is the amount of time it takes for your memory to respond to a command. Specifically, it is the length of time between memory receiving a command to read data, and the first piece of data being sent from the memory.
Unless you are a computer enthusiast looking to push every last bit of performance out of your system, CAS latency needn't be a deciding factor when purchasing memory modules.
You may see CAS latency (clock latency) referred to as CAS#, CL#, CAS=# or CL=#. The performance difference between CAS latency ratings is virtually undetectable.