iPhone Carrier or Operator is a piece of little information which you need for unlock. Carrier is a SIM Card Company who sale iPhone under contract in trade of cheap and locked iPhone. To avail network access on other carriers, it requires to apply for official unlock using websites like iUnlockStore.com but before processing any order, they will require Carrier or Operator Information.
Some known operators are AT&T in United States, where AT&T is carrier company and United States (USA) is country. This information is handy and useful to obtain unlock service. Almost every country has Carrier companies who sale either under contract or without any contract. We will post iPhone carrier companies on this blog, some other day.