the term "unlocked phone" normally means that the phone is not sold or tied to a particular network in any way. these phones are typically sim-card phones (not sure if cdma phones can be unlocked) and may feature one or more cell radio options.
the benefit to such phones is that you can use a phone that your carrier does not sell as long as it is compatible with the radio type/frequency. all that is needed is to put your providers sim card into a compatible phone and you're set.
the good:
-use a phone of your choosing even if not sold by provider
-can switch providers with the phone easily
the bad:
-you pay full price
-you still pay the same for your plan whether you use the new phone discounts or not
-you get ZERO phone support in case of damage, not knowing how to work it, etcetera.
up to you.