Answer Description

The two most common ways to inject pop-up windows into a web browsing session is by using a malicious application which will most likely be found by the anti-virus software, or by installing a plugin or extension to the browser that creates the pop-ups. Some free applications downloadable from the internet will install these plugins as a way to create ad revenue.

Wikipedia

A browser extension is a small software module for customizing a web browser. Browsers typically allow a variety of extensions, including user interface modifications, cookie management, ad blocking, and the custom scripting and styling of web pages.