Here is a brief post on how search engines works & also how your email ID starts getting spammed.
Search bots are network of computers used to index or crawl web pages. Thousands of computers are networked to do this job. So, what exactly crawling means? taking a copy of visited website for future use. Search bots copies all text contents of websites to their particular servers, for ex: googlebot crawls web content to google servers. Many people think that search engines searches on live sites, which is wrong! When a search query is made on search engine, it actually searches on its server which holds the content copied/crawled by search bots. Hence there will be a delay in new pages to come on search results. But Google is superfast, which always tries to give live results. Best example for this is how it indexes Tech Bangalore! Articles on Tech Bangalore gets into Google’s index within 5 minutes of posting!
Google Search also has more functionality than just search. You can use it as currency converter, calculator & many more, complete list of functions are given here http://www.google.co.in/intl/en/help/features.html
Spam bots are similar to search bots, but they crawl email IDs rather than complete web pages! Spam bots visits web pages, looks for email IDs over there, crawls them all to a particular server. These email IDs can be used for spamming in future. You might have seen people mentioning their email IDs on forums in discrete format, ex: prashnath AT techbangalore DOT com. This is just to ensure that, spam bots will not recognize it and avoid crawling. You can also put an image or flash containing your email ID, because both search bots & spam bots cannot read text in image or flash file.