Usually when you get a cell phone from a carrier it is "locked" to that carrier. Basically, this means that a phone you get from Verizon or Virgin, or Rogers (or whoever your cell phone provider is) they program the phone to only work with their service. The phone is "locked" as a Verizon, etc only phone. An unlocked phone is when this programming is removed, and a phone that is "unlocked" can be used with any carrier (with exceptions).
An unlocked phone, is a phone that would work on any network. Regardless of brand and type you can use it. Some would rather getting a Unlocked phone rather than leaving there provider for one that has better phones.
Unlocked phones are more expensive than provider branded phones.
Well if you wanna unlock particular network all you have to do is call up the customer help line of the service provider and ask them for the unlocking code or you can get the unlock code by the third party its take 2-12 hour. I recommend the site http://www.unlock-free.com/ which help you a lot for unlocking