An "unlocked" cell phone means someone has used software to unlock all of the features of the phone.  When certain cell phone service companies buy phones from a manufacturer, the manufacturer programs the phones to that carriers abilities and limitations and also only lets that phone work with that carrier, hence a Verizon Motorola V3i and a Cingular Motorola V3i.  Most unlocked phones operate on GSM networks like Cingular and T-Mobile.  Verizon uses a CDMA network.  SIM cards do not work with Verizon, Nextel, Boost, etc.  SIM cards work with a GSM network like Cingular and  T-Mobile.  Before bidding on an unlocked phone, contact your cellular service company or prospective cellular service company to ask if a particular phone will work on their service.  I have had buyers who purchased an unlocked Razr from me and called Cingular to start their service.  They got their SIM card in 2 days and were up and running.  The difference between that and typical service is they do not have to keep a 2 year service contract with Cingular.  This works for people who want to upgrade their phones ofter without penalties.  Just drop your existing SIM card into the new phone and you are up and running.