What is mechanism behind cloaking?

According to this "Cloaking refers to the practice of presenting different content or URLs to human users and search engines" The same link gives example as:

Serving a page of HTML text to search engines, while showing a page of images or Flash to users

Question: If I had correctly interpreted, there must be a mechanism for identification of the entity whether it is search engine or browser(user) at the web server.What do we call such mechanism? Or is it just a php or JavaScript code that redirects? How does a web-server actually know that entity'X' search engine and entity'Y' is web browser?