Unique visitors are counted according to visits by unique IP addresses. Computers on the internet have IP addresses in a similar way that telephones have unique telephone numbers. It's a little more complicated but this serves for a simple explanation.

A visitor to the site is likely to view several pages during their session. Regardless of how many pages they view, their 'visit' is counted just once. Their session ends one hour after their last click. A session counts as a visit.

This is roughly the number of web pages visitors see. Page views generated by robots, spiders or the processing of components within pages (eg. images) don't count.

A hit is a request to download a file from a server to your computer. Files include web pages or components within them (eg images or form fields). For example, a web page with one image registers as two hits: one hit for the page and one for the image.