A unique visitor is a person or computer (host) that has made at least 1 hit on 1 page of your web site during the current period shown by the report. If this user makes several visits during this period, it is counted only once. Visitors are tracked by IP address, so if multiple users are accessing your site from the same IP (such as a home or office network), they will be counted as a single unique visitor.
Number of visits
The number of visits are the total number of visits by all visitors over a given period of time. If a user visits your site and then comes back 6 more time you should see a unique visit total as 1 and 7 total visits.
HitsThis is every file requested by the visitor. This includes pages and images together. If you have a page with 2 images calling a java script file the page will generate a total of 4 hits. This is the most common referenced stat used and one that is virtually meaningless (and useless). The more appropriate numbers to consider are both ‘number of visitors’ and ‘unique visitors’.
Total number of bytes for pages, images and files downloaded by web browsing. 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.
Source: Understanding AWstats