If you have a cell phone with a SIM card, it is issued by your cell phone carrier and makes your phone compatible with that provider only. Your SIM card helps activate any phone it is inserted into. If you have a locked phone however, the SIM card will only be recognized if it's from your particular cell phone carrier. An unlocked phone will recognize SIM cards from any carrier. Almost all phones are locked, which is really just software that keeps a cell phone exclusive to one carrier.
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Having a locked phone kind of defeats the purpose and one of the benefits of using SIM compatible cell phones. In the U.S. cell phone service providers tend to keep their phones locked since they offer free and/or highly discounted service plans. Locking the phones help them to subsidize any costs.
Due to a lot of complaints, most cell phone carriers will leave their phones unlocked, unlock your phone when you make the request (some for free, others with a fee), or will unlock it after your initial contract with them expires. Even if you have an unlocked phone, you should be aware that your phone many not function fully or correctly with other carriers.
If you purchase a phone from a third party vendor, you can generally get it in its original, unlocked state. However, getting a phone in its original state means you will be paying the full retail price for it (which may not be ideal, especially when you can get it for free or with a huge discount through a service provider).
There are also third party services that will unlock your for a fee. You can also download software that can help you "hack" into your phone and unlock it yourself. However, with both of these routes there is no guarantee that your phone will work properly afterwards and you might even make your phone inoperable.
Cingular Wireless, AT&T and T-Mobile are the only U.S. service providers that use SIM cards and where having an unlocked phone would be ideal. Sprint, Verizon and Virgin Mobile are on CDMA network and do not use SIM cards and unlocking phone from these providers isn't necessary.
