Browser extensions are computer programs that add functionality to existing browsers. They come in as many kinds (and more) as there are browsers. Internet Explorer distinguishes between toolbars and browser helper objects (BHOs) Other browsers like Firefox, Chrome, Opera, and Safari call them add-ons or simply extensions. Other browsers give their extensions less power and are trying different approaches to stop Potentially Unwanted Programs from installing. Removal of browser extensions is usually not difficult in the case of PUPs.”]
Source: https://blog.malwarebytes.com/threats/browser-extensions/