Once you are included in the index, the spider will usually revisit every two weeks. With Excite - these new updates seem to be automatically included once the spider has visited the site. They are submitting a query to a search engine. The search engine, depending on how it has been tuned, will pull up all of the relevant sites which pertain to that query. Better yet, you can very simply include a robots.txt file in your main directory - in this way you can exclude all engines from certain parts of your site, or specific engines from specific pages.
ALL search engines rank pages based on frequency and location of keywords.
The difference between engines and directories.