GSM only works outside the US. The frequency bands (900 and 1800 MHz) are not supported here. So an unlocked SIM can be used in any area where the frequency bands are supported - mainly Europe and the Caribbean, if my memory is correct. Not sure about Canada, but I think they operate in the 850/1900 mHz range.
Here is a map showing coverages: GSM World Coverage Map- GSM Country List by frequency bands
The 8830 is a CDMA device here in the US. If it is a Verizon branded device, then it can only be used on Verizon. If it is a Sprint device, it can only be used on Sprint.