Apps generated an estimated $26 billion in worldwide revenue in 2013, up 44% over 2012, according to Gartner.

The most common way people spent money on those apps was through the "freemium" model -- free downloads that sell items within the app. As of November 2013, the business model accounted for a whopping 92% of the revenue generated in Apple's App Store and 98% of the Google Play app store, according to a year-end report from Distimo.

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