What is the difference between a verizon iphone, sprint and at&t?

I'm thinking about buying an iphone from a us ebay seller. I notice they sell verizon, at&t and sprint. When I get the iphone I plan on unlocking it to put in an irish sim card and would like to know what verizon etc.means and what one I would be best choosing?

The difference is that verizon and at&t provide quality products at competitive rates, while sprint is a bunch of blood-thirsty vampires

AT&T is most common it's a big company. I had AT&T but I unlocked my phone and got GoSmart. I would say get either sprint or Verizon. But if I had to chose between those three it's not AT&T or Verizon. So if I was you is go with Sprint. But all of them make you sign a contract/agreement. They also have T-Mobile if you want that one. But go with Sprint.

Not all iphones can be unlocked. I suggest a att one. All those are easy to unlock

The difference between the carriers is the types of networks that they support.

AT&T is a GSM carrier which means it can be used in other countires like Europe, Asia, etc because it uses a SIM card

Verizon & Sprint are CDMA they don't use SIM cards and are able to be used in the United States only