what does "unlocked GSM" mean for cell phones???

Answer 1

That means you can use the phone with any wireless provider such as AT&T, T-mobile, Cingular, etc. In the previous years, a phone issued by a wireless provider company could be activated by the same company.
Though the phone can be activated using different companies, it is not necessarily can be used worldwide.

Answer 2

If you see a cell phone that says unlocked GSM that means that cell phone can be used with Cingular (A T&T), T-Mobile or Nextel. GSM network requires all cell phones to have a sim card on the back of that phone.

Answer 3

a GSM that is not locked