It means you can use it with any cell phone provider. Usually phones are made for a specific provider (like AT&T or T-mobile, for example). But an unlocked phone means you don't need a specific provider- it should work with any cell phone company/contract.
Wireless internet can work using one's cell phone by making sure that one's phone is web-capable or have a data plan from cell phone provider. One can therefore use the cell phone as a modem,making it possible for the phone to go online on its own.
Any cell phone that is unlocked requires a SIM card to work. If you own a Sidekick cell phone that is unlocked you are able to use a carrier other than T-Mobile. For this reason, it will require a SIM card to work.
No, because the cell phone networks work on different frequencies, there is no way to get a cell phone bought in Japan to work here. If it was manufactured in Japan, but intended for sale in the United States then it would work, but not if it was intended for sale in Japan.
Go to your inventory, then click the cell phone icon on the side near the selling space. That gives you access to your cell. Also, you can click on your sim and click 'Cell Phone' to access your cell. Either way will work.