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.

Pages - This is the total number of pages viewed by visitors. This does not include images, javascript or CSS and the like. Just HTML and CGI type files.

Hits - This is every file requested by the visitor. This includes pages and images together. If you have a page with 2 images calling a javascript file the page will generate a total of 4 hits. The most commonly 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' (see above).

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.