Difference between GSM and Global iPhone 5
The CDMA model presumably has compatibility in more markets than the standard GSM model.  According tot he list below, unless you ordered your phone from a Chinese firm, if you have a CDMA model it also supports GSM.  

GSM is a European standard and supports the EDGE network.  CDMA supports some networks branded as "3G", but YMMV.  Both are getting a bit long in the tooth as far as cutting-edge goes (IMO).

You can use this list to help identify if your model includes CDMA (Global) or GSM:
http://support.apple.com/kb/ht3939

Check the "Model" at the bottom on the back of the phone and cross-reference it with the model number on that list. Here are the relevant iPhone 5 data:


GSM models: A1428, A1533 and A1530
CDMA models: A1429 and A1453
CDMA China models: A1528, A1530 and A1518