@twitter internationally expanded the use of its app-install ads - ads that appear in Twitter’s news feed (targeted by interest, keywords, gender, language, geography, etc.) that include links to download apps directly from Apple’s App Store and Google’s Play Store. Twitter has also introduced a new pricing model for these ads. To ensure the highest possible return on investment, Twitter will be using a cost-per-app-click model. Thus, an advertiser will only be charged when a user clicks on its app-install ad. - Link