Duolingo is a free to use language-learning app that aims to bridge the internet language barrier. What do I mean? Well let's start with the basic fact that thousands or articles are published every day all over the world in different languages. Yes there are internet translating services, but things quite often get lost in computed translation. How do we fix that? Have real people do the translating!
Okay, okay... here's a real life example. Lets say you're a native French speaker who wants to learn english. Duolingo starts by feeding you a line from an English website and then asks you to translate it. That sounds impossible, but Duolingo starts by giving you very simple sentence that they help you translate. Then they help you memorize those new words by giving you educational examples.
It's a genius business model with wonderful intentions. I personally can't wait to start using what Spanish I do know to help translate the internet. With time hopefully one day I'll be able to fluently translate this one! :p