When users visit a website (via a user agent such as a web browser), a text string is generally sent to identify the user agent to the web server. It is known as the "user agent string". The Netscape web browser identified itself as "Mozilla/<version>" followed by some information about the operating system it was running on.

Because the Netscape browser initially implemented many features not available in other browsers and quickly came to dominate the market, a number of web sites were designed to work, or work fully, only when they detected an appropriate version of Mozilla in the user agent string. Thus, competing browsers began to emulate ("cloak" or "spoof") this string in order to also work with those sites. The earliest example of this is Internet Explorer's use of a user agent string beginning "Mozilla/<version> (compatible; MSIE <version>...", in order to receive content intended for Netscape, its main rival at the time of its development. This format of user agent string has since been copied by other user agents, and persisted even after Internet Explorer came to dominate the browser market.