What Is a Software Spider (or Robot)? : What is a Search engine spider or search engine robot
A "software spider" is an unmanned program operated by a search engine that surfs the Web just like you would. As it visits each Web site, it records (saves to its hard drive) all the words on each site and notes each link to other sites. It then "clicks" on a link, and off it goes to read, index and store another Web site.
The software spider often reads and then indexes the entire text of each Web site it visits into the main database of the search engine it is working for. Recently many engines such as AltaVista have begun indexing only up to a certain number of pages of a site, often about 400, and then stopping. Apparently, this is because the Web has become so large that it's unfeasible to index everything.
How many pages the spider will index is not entirely predictable. Therefore, it's a good idea to specifically submit each important page in your site that you want to be indexed, such as those that contain important keywords.
A software spider is like an electronic librarian who cuts out the table of contents of each book in every library in the world, sorts them into a gigantic master index, and then builds an electronic bibliography that stores information on which texts reference which other texts. Some software spiders can index over a million documents a day!
What the spider sees on your site will determine how your site is listed in its index. Search engines determine a site's relevancy based on a complex scoring system that the search engines try to keep secret. This system adds or subtracts points based on things like how many times the keyword appeared on the page, where on the page it appeared, and how many total words were found. The pages that achieve the most points are returned at the top of the search results, the rest are buried at the bottom, never to be found.
As a software spider visits your site, it notes any links on your page to other sites. In any search engine's vast database are recorded all the links between sites. The search engine knows which sites you linked to, and more importantly, which ones linked to you. Many engines will even use the number of links to your site as an indication of popularity, and will then boost your ranking based on this factor.