An ad blocker is a software product that prevents advertisements from appearing with the content the user is intentionally viewing.
People block ads for a variety of reasons. For example, many of them find interrupt marketing ads annoying and even stressful. Interrupt marketing is intrusive by design, like an interstitial ad that comes between users and the content they are attempting to view or an autoplay or rollover ad that starts up without any intentional act on the part of the user. Other issues that lead people to block ads include a lack of relevance to user interests and security concerns associated with malvertisements.
Nevertheless, online ads are often considered a necessary evil. They provide much of the revenue required to support online content, which the user generally expects to access free of charge. However, ads can also degrade the user experience, in which case people may either stop visiting the site or install an ad blocker -- neither of which is a good outcome for the site owner.
The way ad campaigns are run is partially to blame. Commonly, marketers are paid based on the number of impressions an ad receives, simply the number of people who see it. The visitor's response to the ad is irrelevant in that context. Approaches designed to be less intrusive and more geared toward the user include discoverability marketing, contextual marketing and permission marketing.
Another approach involves improving the appeal of the ads themselves, so that users are less tempted to avoid seeing them. The viral ad, which might be considered the holy grail of marketing, is based on understanding the target audience and providing them with something of value.