Unique Visitor: Unique visitors are determined by the number of unique IP addresses on incoming requests that a site receives.

Visits: Number of visits made by all visitors. So they may view several pages and generate lots of hits, but are counted once.

Pages Viewed: This is roughly the number of HTML pages or scripts that visitors have viewed (a "page" is a "hit" that is not an image file and was not viewed by a "spider" or "robot").

Hits: A "Hit" is a term used that represents the number of files downloaded from a server to your computer. For example, if a page has ten different graphics, and someone visited it, it would count as eleven hits, one for the page load, and ten for all the different graphics.