A file which contains important information about the accesses to Web pages made by users. For example, every time a browser requests a page, an entry is automatically made in this file by a Web server, containing information such as the time at which the access was made, the address of the computer on which the browser was running, and the transfer time of the page. Such information is exceptionally useful. For example, a Webmaster is able to use it to judge whether certain pages should be kept in a fast storage medium in order to optimize them. A company is able to use the information to judge which pages are popular and which pages are hardly visited. They can then adjust marketing tactics and the structure of their Web site. Web log files are also increasingly being used to monitor employees' use of the Web: for example, to detect those who visit computer game sites during office hours. The development of software tools for analysing log files has become a major business, and such tools are often referred to as Web logging tools. There are a number of formats which define the contents of Web log files: what data is in the file and the order it appears. The most common is the Common Logfile Format which was defined by the National Center For Supercomputing Applications. Microsoft have also defined a similar format known as the IIs Standard Log File Format which is associated with their IIS server. The World Wide Web Consortium have also defined a format which is particularly suited to analysis, and this is known as the Extended Logfile Format. There is also an experimental format known as the Extensible Log Format based on XML. The aim of the developers of this format is to standardize log file formats over every Web server.