All web-site visitors can be divided into users and robots. Robots are search bots. Users are people working with site by means of a browser.
Robots are singled out with the help of "Robots" data dictionary. If a visitor's User-Agent record appears in this dictionary, then the visitor is considered as robot, otherwise he is counted as a user.
Users uniqueness is defined in various ways according to the counter type. When php-include is chosen for a counter, then a user (IP-address) considered unique if he didn't download the site pages for more than an hour. In case other counter types is used, then cookie is enabled, which expires at 23.59.59. As long as cookie is enabled, the user is not counted as a new one.