If you reboot a phone does it reset the whole phone?

Assuming you're talking about a 'factory reset' - yes. A factory reset removes any personal data, and reverts the handset to the state it was when it left the manufacturers. The phone companies buy the phones in the factory-set state, and add their own software packages before selling to the public. If you perform a factory reset - all the wall-papers, ringtones, contacts, pictures, music and videos that you've downloaded or copied across to your phone - will be erased.

A standard reboot, however, will not reset your data. You should actually reboot your phone regularly to keep it working as well as it can.