An unlocked phone means it can be used on any carrier, not just the one it was originally sold to. If it is locked, then it can only be used on the carrier that it is locked to.
No. An unlocked phone means it is not branded by a cell phone carrier. This means you can pick a supported cell phone carrier and take your phone there. For example, if I'm ever not satisfied with Straight Talk I can take my phone and go to another supported network.
Think of a jailbroken phone like this: instead of a guest account on the phone, you get an admin account on the phone.