Nowadays, WWW (World Wide Web) becomes more popular and user friendly for transferring information. Therefore people are more interested in analyzing log files which can offer more useful insight into web site usage. Web usage mining is one of the data mining fields, which deals with the discovery and extract useful information from web logs. There are three phases in web usage mining, pre-processing, Pattern Discovery and Pattern Analysis. This paper describes the importance of pre-processing methods and steps involved in retrieving the required information effectively.