Files:
Some requests made to the server, require that the server then send something back to the requesting client, such as a html page or graphic image. When this happens, it is considered a 'file' and the files total is incremented. The relationship between 'hits' and 'files' can be thought of as 'incoming requests' and 'outgoing responses'.

Pages:
Pages are, well, pages! Generally, any HTML document, or anything that generates an HTML document, would be considered a page. This does not include the other stuff that goes into a document, such as graphic images, audio clips, etc... This number represents the number of 'pages' requested only, and does not include the other 'stuff' that is in the page.

Sites:
Each request made to the server comes from a unique 'site' (or person), which can be referenced by a name or ultimately, an IP address. The 'sites' number shows how many unique IP addresses made requests to the server during the reporting time period. This DOES NOT mean the number of unique individual users (real people) that visited, which is impossible to determine using just logs and the HTTP protocol (however, this number might be about as close as you will get).

Visits:
Whenever a request is made to the server from a given IP address (site), the amount of time since a previous request by the address is calculated (if any). If the time difference is greater than a pre-configured 'visit timeout' value (or has never made a request before), it is considered a 'new visit', and this total is incremented (both for the site, and the IP address). The timeout value for our servers is 30 minutes, so if a user visits your site at 1:00 in the afternoon, and then returns at 3:00, two visits would be registered.

KBytes:
The KBytes (kilobytes) value shows the amount of data, in KB, that was sent out by the server during the specified reporting period. This value is generated directly from the log file, so it is up to the web server to produce accurate numbers in the logs (some web servers do stupid things when it comes to reporting the number of bytes). In general, this should be a fairly accurate representation of the amount of outgoing traffic the server had, regardless of the web servers reporting quirks. (A kilobyte is 1024 bytes)

Top Entry and Exit Pages:
The Top Entry and Exit tables give a rough estimate of what URL's are used to enter your site, and what the last pages viewed are. Because of limitations in the HTTP protocol, log rotations, etc... this number should be considered a good "rough guess" of the actual numbers, however will give a good indication of the overall trend in where users come into, and exit, your site.