What does GSM Unlocked mean on E-bay?

Question

I recently broke my phone and I want to buy a new one off e-bay. How do I know if the phone is compatible with my service and what does GSM unlocked mean?

Answer 1

it means that it is not "network locked" to a specific carrier, you can use any network provider of your choice.  

Answer 2

GSM UNLOCKED means that the phone uses a chip and that i can be used with any network that requires a chip.