Internet is becoming to be the largest info base although the web is full of a mass of pages . it is the great problem facing to the information scientists that how to make people obtain the information quickly and accurately Tnternet已经成为目前世界上最大的信息资源库,但是网上信息资源纷繁芜杂,如何满足人们对快速、准确而全面获取信息的要求,已经成为摆在人们面前的一大难题。