Ad popups - Separate windows with ads based on your browsing history or even whatever they find installed on your system.

Ad injection - Putting ads on web pages they didn't write, such as Google search results.

Ad substitution - Replacing ads on web sites with their own ads.

Cookie stuffing - Injecting or clobbering referral cookies for sites like Amazon so that they get an undeserved commission on things you buy.

They sell traffic to Pay Per View companies, which in turn sell it to affiliates. Basically, if you are an affiliate you can go to a PPV company and buy pageview for a landing page of your choice which will be prompted in response to a given action by the user. For example, you might want to prompt your scammy acai berry trial affiliate page to people who searched for the word 'diet' in google or visited weightwatchers.com. The ad software takes care of popping up the right ad for the right user, according to the criteria of the campaign.

Also search - replacing the browser's default search engine with their own or a partner's and getting a cut of the ad revenue.