CAS stands for Column Access Strobe, so CAS latency is the Column Access Strobe latency. Without going into too much detail, its the amount of time it takes for your memory to access a chunk of data, measured in clock cycles. CAS 2 and 3 are simply speed ratings of memory, that says taht for a given frequency, the memory can access a chunk of data within 2 memory clock cycles(not CPU clock cycles).
Normally you can check within your BIOS to see if you have CAS 2 or 3 enabled. Other latency values are RAS to CAS latency and precharge timing.