The IP address of a user visiting a website combined with another piece of data (usually a cookie) are used to determine that a user is a unique user. This data is important as a site with one hundred regular visitors who come fifty times a day to chat might be less valuable to an advertiser than a site with five thousand unique users per day as the second site has considerably greater ‘marketing reach.’
Unfortunately there are a huge number of factors which cloud judgement of this figure. If you visited this site today this morning at home, then at work then on your Android tablet on the way home from work you would be identified by three unique IP addresses and three unique browsers. As a result the site would appear to have three unique visitors. When you combine this with the users who regularly clear cookies it starts to further distort the number even when measured over such short periods as one month. When you are planning your advertising campaigns it’s worth bearing in mind that a site with two thousand unique visitors per day may have half that figure or less in reality.