Well, there is more than one type of search engine technically. There are those such as Google that automatically crawl through the internet and there are human powered directories (Humans still have their uses and the Open Directory Project is an example, where real people review a listing)

To find information on the millions of Web pages that exist, a search engine uses special software robots, called spiders, to build lists of the words found on Web sites. The process is called 'crawling'. There are three very hungry spiders on the Net. Their names are Googlebot (Google), Slurp (Yahoo!) and MSNBot (MSN Search)

That might create a nice picture in the mind, but in fact these spiders don't actually go anywhere. They find web pages by following links in pages that they already know about. They are not clever enough to type in a web address (yet!)

Whis means of course is that if your website is not linked with another website, the spider won't see it - unless a human being lets the search engine know you're out there.

After spiders find pages, they pass them on to another computer program for "indexing." This program identifies the text, links, and other content in the page and stores it in the search engine database's files so that the database can be searched by keyword.

Every time you change your web pages, there's a time lag before a crawler based search engine finds the change, and of course that change can affect your listing.

Ok, it's a bit more complicated than that - Each search engine has developed a set of rules and mathematical equations, known as an algorithm, which it uses to set the order of its rankings. If you are interested there are plenty of websites that will explain this to you!

To get onto the search engines then, you either have to tell the search engine you are there, or have other websites link to you. Beyond that, there are plenty of websites offering you a quick fix to the problem!