Unique Visitor:
A unique visitor is a host that has made at least 1 hit on 1 page of your web site during the current period shown by the report. If this host make several visits during this period, it is counted only once.
The period shown by AWStats reports is by default the current month.
However if you use AWStats as a CGI you can click on the "year" link to have a report for all the year. In a such report, period is full year, so Unique Visitors are number of hosts that have made at least 1 hit on 1 page of your web site during those year.
 
Visits:
Number of visits made by all visitors.
Think "session" here, say a unique IP accesses a page, and then requests three others without an hour between any of the requests, all of the "pages" are included in the visit, therefore you should expect multiple pages per visit and multiple visits per unique visitor (assuming that some of the unique IP's are logged with more than an hour between requests)
 
Pages:
The number of "pages" logged. Only files that don't match an entry in the NotPageList configure parameter (and match an entry of OnlyFiles config parameter if used) are counted as "Pages". Usually pages are reserved for HTML files or CGI files, not images nor other files requested as a result of loading a "Page" (like js,css... files).
 
Hits:
Any files requested from the server (including files that are "Pages") except those that match the SkipFiles config parameter.
 
Bandwidth:
Total number of bytes downloaded.
 
Entry Page:
First page viewed by a visitor during its visit.
Note: When a visit started at end of month to end at beginning of next month, you might have an Entry page for the month report and no Exit pages.
That's why Entry pages can be different than Exit pages.
 
Exit Page:
Last page viewed by a visitor during its visit.
Note: When a visit started at end of month to end at beginning of next month, you might have an Entry page for the month report and no Exit pages.
That's why Entry pages can be different than Exit pages.
 
Session Duration:
The time a visitor spent on your site for each visit.
Some Visits durations are 'unknown' because they can't always be calculated. This is the major reason for this:
- Visit was not finished when 'update' occurred.
- Visit started the last hour (after 23:00) of the last day of a month (A technical reason prevents AWStats from calculating duration of such sessions).
 
Grabber:
A browser that is used primarily for copying locally an entire site. These include for example "teleport", "webcapture", "webcopier"...
 
Add To Favorites:
This value, available in the "miscellaneous chart", reports an estimated value of the number of times a visitor has added your web site into its favorite bookmarks.
The technical rules for that is the following formula:
Number of Add to Favorites = round((x y) / r)
where
x = Number of hits made by IE browsers for "/anydir/favicon.ico", with a referer field not defined, and with no 404 error code
y = Number of hits made by IE browsers for "/favicon.ico", with a referer field not defined, with or without 404 error code
r = Ratio of hits made by IE browsers compared to hits made by all browsers (r <= 1)
 
As you can see in formula, only IE is used to count reliable "add", the "Add to favorites" for other browsers are estimated using ratio of other browsers usage compared to ratio of IE usage. The reason is that only IE do a hit on favicon.ico ONLY when a user add the page to its favorites The other browsers make hits on this file also for other reasons so we can't count one "hit" as one "add" since it might be a hit for another reason.
AWStats differentiate also hits with error and not to avoid counting multiple hits made recursively in upper path when favicon.ico file is not found in deeper directory of path.
 
HTTP Status Codes:
HTTP status codes are returned by web servers to indicate the status of a request. Codes 200 and 304 are used to tell the browser the page can be viewed. All other codes generates hits and traffic 'not seen' by the visitor. For example a return code 301 or 302 will tell the browser to ask another page. The browser will do another hit and should finally receive the page with a return code 200 and 304. All codes that are 'unseen' traffic are isolated by AWStats in the HTTP Status report chart, enabled by the directives ShowHTTPErrorsStats. in config file. You can also change value for 'not error' hits (set by default to 200 and 304 with the ValidHTTPcodes directive. The following table outlines all status codes defined for the HTTP/1.1 draft specification outlined in IETF rfc 2068.
They are 3-digit codes where the first digit of this code identifies the class of the status code and the remaining 2 digits correspond to the specific condition within the response class. They are classified in 5 categories:
 
1xx - informational
2xx - successful
3xx - redirection
4xx - client error 
5xx - server error