Search engines regularly collect information about new web sites. A programme called a robot or spider or crawler checks the pages of sites it finds. With the help of another programme it then creates a catalogue describing all the pages it finds, listing each page name with various appropriate keywords. When surfers ask the search engine for a document they enter keywords. The search engine compares these keywords with those in its catalogue and then sends the user a list of suitable matching links.
Directories are like online yellow pages.
Sites are listed under such categories as Art, Science, Shopping etc. Many directories also offer search engine facilities, but these search only pages listed in the directory. Directories do not send out robot to find and index new pages. Owners of new page have to submit their URL to be listed. Portals are the principal starting sites which surfers connect to when they start to surf the Web.