CDMA and GSM are the different types of networks. For example, Verizon uses CDMA and AT&T uses GSM. The phones between each network aren't interchangeable as they use different radios.
I'm not familiar with the Trophy, but I'm assuming CDMA/GSM, means the variety available. Unless it has both CDMA and GSM capabilities, then it can be used on both types of networks.
And as for the SIM card, you can get one from your service provider. Easiest to walk into a corporate store and pick one up.
I went from the Droid 1 to the HTC trophy almost a year and a half ago. The Verizon trophy is cdma and doesn't require a SIM card for using in the US. It is global capable and requires a SIM for overseas use.
