Can someone explain the difference between thes two (simply please) and tell me which one is in use in Thailand?

CDMA - Code Division Multiple Access describes a method of transmission of data. UMTS - Universal Mobile Telephone System is a far more "generic" term (also known as "GSM"). Both refer to telephone standards. CDMA is (predominantly) American while UMTS is more global.

CDMA uses the 850MHz band and UMTS uses 1900MHz

To make things more confusing CDMA and especially W-CDMA are often also branded as UMTS, and practically everything is branded as 3G.

Thailand has licencees on all the frequency bands - so technically "both" would be the answer to which is in use in Thailand.

Fwiw, Truemove-H, My by CAT, DTAC3G are all on 850MHz.

Why are you asking? Do you want to know technical details, like in the above? Wikipedia has all the gory details.

In Thailand the only CDMA network is CAT's CDMA. It's slower and less capable than the 3G networks of AIS and True-H. It's also less flexible as there are no SIM cards - devices must be purchased from CAT and are bound to its network (which CAT has said it would abandon although it's still running). So CAT CDMA cannot be recommended. True-H or AIS for 3G, both offer cheap dongles and competitive plans.

Thanks. Just what I wanted to know.