It is a software application or online service that displays adverts of some kind while it is running in order to provide revenue for its owner.
Sometimes developers will offer a 'free' version of their software on the condition you have to view adverts, they get paid by the number of people clicking on the ads. Quite often there is also a paid version of the same software that is advert-free.
A darker use of the term adware is as a form of malware where users are presented with adverts in uncloseable windows or pop-ups that cannot be stopped.
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Click on this link: Adware