Apps are small programs available on mobile devices that fall into categories like games, social networking, books, etc. Most apps cost between free and $2, with games being the most popular category downloaded. While app downloads and engagement are initially high across categories, they generally drop off steeply over time except for entertainment and lifestyle apps. Free apps see lower engagement rates over time compared to paid apps, though the rates even out. Android is gaining rapidly in available apps and downloads compared to iOS as its app store becomes more open.