Say what? OK, this is a little advanced but it's kind of cool so I'm posting it. :-)

The user agent is the fancy name for your browser. The reason that it is important is that it tells the web server what kind of browser you're using. Some sites actually will serve different files based on what your user agent is. For example, you Mac users (like me), have you ever been to a software site that offered you the Mac version to download without you having to tell it that you're on a Mac? That's because they detected your user agent.

There was an article recently on Forbes.com called "Condemned To Google Hell" about companies who have been banned from Google. If you just read the first few paragraphs, you might think to yourself that Google just arbitrarily baned their websites. But near the bottom, you read this:

he admits that he tried gray-area tactics like buying links from more established sites to juice his traffic.

and

...[the owner] thinks he can trace his problem to...replicated content on many of the site's pages, making them look like duplicate--that is, spam--content.