Cloaking

Cloaking means showing a search engine a different page than you show a visitor. It breaks Google's rules and can get you a penalty, which is why it counts as a black hat technique.

How does cloaking work?

Your web server can easily recognize spiders such as Googlebot, because they send the name Googlebot in their user agent. Your web server, or any scripting language, can read the user agent. Based on it, you can set up your script to serve a different page.
You could, for example, show Google more links and fewer ads, so Google thinks your page is more user-friendly than it really is.

Why is cloaking bad for your site?

Cloaking is cheating and falls under black hat SEO. Google sees one version of your site and sends visitors to your page based on that version, but those visitors get the other one. Google treats this as spam and will eventually give your page a penalty for it. A penalty like that means you practically disappear from the search results. So stay away from cloaking.
You are much better off optimizing your pages and content for both visitors and search engines. That keeps your site ranking well for longer.

Advanced techniques

Cloaking as described above is easy for Google to detect. Another form of cloaking serves content that only a real browser will ever see. Take scrolling. You scroll on almost every website, but a bot like Googlebot has no need to. With JavaScript you can add extra content to the page as soon as a user scrolls, and Googlebot will never see it.