What is the difference between a "locked" and an "unlocked" cell phone?

What is the difference between a "locked" and an "unlocked" cell phone?

TJ McClearin (Business Manager, Infinity Wireless, LLC) gives expert video advice on: How do cellular phones work?; What is the difference between a "locked" and an "unlocked" cell phone? and more...

With cell phone basics, the difference basically between an "unlocked" and "locked" cell phone is that a locked phone comes directly from the carrier. So, Carrier A has the phones that they test, set up and have already preconfigured to talk to their network and transmit data with and whatnot. Whereas an unlocked phone however, is directly from the manufacturer, it comes from Motorola and Nokia. The carriers don't ever touch them, because of that, they can use whatever SIM card they want inside, whereas a locked phone can only take Carrier A's SIM card, or, a Carrier B locked phone can only take a Carrier B SIM card. In contrast, an unlocked phone can take either/or SIM card.