i am looking into getting a sidekick phone from ebay but im not sure about some things, what does it mean if the phone is unlocked, it it better to buy one which is locked or unlocked? i now live in the UK, if i bought the phone from a seller in the US does that mean that i cant use it here?

also, the phone comes without contract, would i be able to take out a contract in the UK for this phone e.g. with T-mobile or would i be able to put my sim card into the phone when it arrived and start using it?

any help would be very much appreciated, thank you