Since you don't have a location listed, I'm going to assume you're in the US. Here, Verizon and Sprint are the two major CDMA carriers. T-Mobile and AT&T are the main GSM carriers. The biggest difference (without getting into the technical side) is that GSM carriers use a SIM card (a small chip that contains your phone number and links with your phone account at T-Mo/AT&T) and CDMA carriers do not use one. The advantage of GSM is because the information is stored on the SIM card, if you buy a new phone then you can switch the SIM into the new phone and be good to go. With CDMA, you have to either log into your account or call Verizon/Sprint and have the service switched to the new phone. And since GSM is more widely used around the world, if you get your handset carrier unlocked, then rather than paying roaming fees when abroad, you can just pop a local SIM and be good to go.
Also, the link Interstella5555 posted has some more good info if you go with one of the first three links.