PHONE LOCK / SIM LOCK:

A SIM lock, not to be confused with PIN or PUK code, is a capability built-in to GSM phones by mobile phone manufacturers. 
Network providers use this capability to restrict the use of these phones to specific countries and network providers. 
Currently, phones can be locked to accept only SIM cards from one or more of the following: 
-Countries (the phone will work in one country, but not another) 
-Network/Service providers- 
-SIM types (i.e. only specific SIM cards can be used with the phone). 

UNLOCKING: 

A handset can be unlocked by: 

-Entering Unlocking Code 
-Unlocking Sofware [Via USB Cable or Bluetooth] 

Unlocking Softwares are readily available on the Internet, too many to mention!