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What is a hit or request?
A hit is the result of a file being requested and served from your web site. This can be a html document, an image file, an audio track etc. etc. Web pages that contain a large number of elements will return high hit scores. Hits are of very little consequence when analyzing your visitor demographics.
What is a page view?
A page view means just that. Once again, it is not a true indication of how many different people are visiting your web site, but it is a good way to judge how "sticky" (the ability to retain the interest of visitors) your web site is and is an important consideration regarding the possibility of attracting high paying advertising.
What is a unique visitor?
A unique visitor is where stats really count. It is someone with a unique IP address who is entering a Web site for the first time that day (or some other specified period). When you log onto the Internet, you are assigned a unique IP address, or if you are a cable modem user your IP address is usually "static", meaning that it never changes.
Your IP address is an identifier and while you are using it, no one else on the Internet can utilize that particular set of numbers. Your number is counted once, usually for a 2-24 hour period, dependent upon the tracking software.
So no matter how many times a visitor refreshes or navigates through your web site, they will only be counted once for the specified time period. This is by far the more accurate way of analyzing web site performance.
Note: the notion of a "unique" visitor as described above can be somewhat misleading in the instances of multiple users sharing the same IP. For example, the proliferation of home networks in recent years may see you having 2 or more people from the same network accessing your site via the same originating IP. This can also occur in the cases of universities accessing your site. In these circumstances, it is more difficult to tell if 1 person from the same IP is viewing 50 pages, or if it's 10 people.