As the author of the WWW FAQ, I regularly answer questions about the workings of the Web. If a question is frequently asked, I simply add an article to the FAQ. But sometimes a question is more detailed, more in-depth— not really a FAQ, but still of interest to others. You'll find those questions, with my answers, here in Innards along with commentary on other web-technology-related topics.

Q. How do websites like realtor.com or eBay work?

A. Those are two very different sites. They have one major thing in common, though: they aren't just collections of prewritten web pages. They generate their content "on the fly," drawing information from databases and collecting more information from the user to store in those databases.

So, to answer your question:: they use PHP, ASP and other server-side dynamic web programming languages in order to generate content on the fly, and not just deliver "canned" web pages.

Note that any particular site might choose to use any of the server-side programming languages. Which one they choose doesn't really matter, because what finally reaches the web browser is just plain HTML (and images, Flash, et cetera).

See the dynamic web development questions section of my WWW FAQ for more information on how to build web sites that interact with users and databases.