Web analysis is primarily done to measure how a website is doing or how it can perform. The motivation of many website owners to do web analytics is mainly to analyze the results of their marketing campaigns. In order to know whether such efforts are giving them good returns, they have to know how it is helping them bring in sales.
One main indicator used to measure performance is the Visitor / Unique Visitor / Unique User measure. This refers to every unique client making requests on the server or each unique client viewing several pages on the site in a given defined period of time like in a day, in a week or in a month. Within this given time period, one and the same user will be counted as one unique user. He may visit the site again and view several pages at a time but he will still be counted as one unique visitor.
The time period is the operative word. Again, multiple visits from the same visitor in a given time period are counted as one. After which he would be counted as another for a certain time period again. It is important to understand that the term unique user or visitor is merely a term for measurement and it does not necessarily mean the individual himself.
Measuring the number of unique users or unique visitors can be done simply by counting the number of usernames, again, in a given time period. That is the reason why many websites enforce all of its users to login. Some would even let guests login as guests just to get a more accurate measurement.