Cloaking refers to the practice of presenting different content or URLs to human users and search engines. Cloaking is considered a violation of Google's Webmaster Guidelines because it provides our users with different results than they expected.
Some examples of cloaking include:
- Serving a page of HTML text to search engines, while showing a page of images or Flash to users
- Inserting text or keywords into a page only when the user agent that's requesting the page is a search engine, not a human visitor
If a site gets hacked, it's not uncommon for the hacker to use cloaking to make the hack harder for the site owner to detect. Read more about hacked sites.