A page hit will be recorded any time a visitor loads a page containing the stats code. Page hits are recorded in their own log by hour, day, month, and year.
The unique visitor report is tracked independently of the other stats reports. Unique hits are recorded for the hour, day, month, and year. If a visitor comes to the site for the first time ever, a unique hit is logged for the year, month, day, and hour. If the visitor loads a page on the site again and the hour has changed, a unique hit for the new hour is logged. Simillarly if if the day, the month, or the year changes a new unique hit will be logged. To record unique visitors, cookies are currently required. If a visitor clears their cookies or uses another browser, they will be counted as a new unique hit. Non-cookie visitors are not currently counted as unique visitors, although extrapolation of unique visitors could be done by using the "Vistor Profile Cookies Report".
Raw Logs/Popular Pages
For each page hit, basic information such as referrer, ip address, and page name is recorded in a raw log of information and is kept for a specified number of days, depending upon the stats package purchased. Beyond this number of days, the information is agregated and stored in summary form.
Visitors/Referrers/Paths and most of the remaining stats types are collected on a per visitor basis. The stats system uses a combination of cookies and logs to determine when a visitor has come to the site, how long they stay on each page, what path of pages they follow while on the site, and when they leave. A visitor begins a new path when they have no previous cookie, when the referrer for that page is from another site, or when their session has timed out. In order to determine how long a user stays on the site, the stats code will periodically re-submit a call to the server. If the server receives this call, or if the visitor loads another page from the site, the server assumes that the visitor is still on the same path.
Because visitor information is collected only when the visitor has cookies, it is necessary to base cookies off of "non-visit" information. Therefore the cookie report is based off of the total number of page hits rather than the number of visitors. It should be possible, using the numbers from the cookie report (the number of page hits with and without cookies), to estimate the number of non-cookie visitors using the total visitor count from another report.