The spider will periodically return to the sites to check for any information that has changed. The frequency with which this happens is determined by the moderators of the search engine.
A spider is almost like a book where it contains the table of contents, the actual content and the links and references for all the websites it finds during its search, and it may index up to a million pages a day. Some examples are Excite, Google, Lycos, and AltaVista.
When you ask a search engine to locate information, it is actually searching through the index which it has created and not actually searching the Web. Different search engines produce different rankings because not every search engine uses the same algorithm to search through the index.
One of the things that a search engine algorithm scans for is the frequency and location of keywords on a web page, but it can also detect artificial keyword stuffing or spamdexing. Then the algorithms analyze the way that pages link to other pages on the Web. By checking how pages link to each other, an engine can both determine what a page is about, if the keywords of the linked pages are similar to the keywords on the original page.
