cas, simple said, is a part of the timing-specs (latency) of the memory. Lower cas means faster memory access (usually also more expensive memory.) Common cas value for PC3200 ram is 2.5.
Btw. WHY would you want to buy a $107 CAS 3 stick of 512MB ram ? I would rather buy CAS2.5 corsair value ram for $77 from newegg or so