Unique Visitors - These are the total number of visits by a unique IP address. This can be a bit misleading because dial-up visitors get a new IP each time they log on so you can have the same person visit different times and give a unique hit.
Number of Visits - The number of visits is the total number of visits by all visitors over a given period of time. If I visit your site and then come back 4 more times you should see one Unique visit and 5 visits from me.
Bandwidth - The total number of bytes downloaded. If you have a page that has 50 KB of text, 2 images at 24 and 32 KB then each visitor to that page will take 106 KB of your bandwidth.