Adware is unwanted software that is designed to display advertisements, usually within a web browser. It uses shady methods to either disguise itself as legit or piggyback on another program to get installed on your computer. Once installed, it changes the way your browser behaves by injecting ads into web pages, causing pop-up windows or tabs to open, and changing your home page or search engine. The makers of adware are sometimes doing so behind closed doors, working as professional black hat hackers or with organized crime units.”]
Source: https://blog.malwarebytes.com/101/2015/07/faqs-about-mac-adware/