This blog posting applies to Microsoft Office SharePoint Server 2007.
The site usage reports that MOSS provides are fairly simple, but can be useful if you haven't got any other tools for tracking site usage. One problem with them, however, is that the information they present is not really explained anywhere. Columns are often named "Value" and sometimes it's not clear exactly what the value represents. Another example is what does "Requests" actually measure? The answer is that Requests always measures Page Views, not all HTTP requests for individual items like images, style sheets, etc.
The attached document gives clear, detailed explanations of all out-of-box report elements.