The Mixxer is a free educational site for language learners and teachers to find a language partner for a language exchange. The language partner is someone who speaks the language you study as their native language and is studying your native language. The partners then meet online to help each other practice and learn a foreign language.

Today, we build complex software applications based on the things computers do well, such as storing and retrieving large amounts of information or rapidly performing calculations. However, humans still significantly outperform the most powerful computers at completing such simple tasks as identifying objects in photographs—something children can do even before they learn to speak.

When we think of interfaces between human beings and computers, we usually assume that the human being is the one requesting that a task be completed, and the computer is completing the task and providing the results. What if this process were reversed and a computer program could ask a human being to perform a task and return the results? What if it could coordinate many human beings to perform a task?

Amazon Mechanical Turk provides a web services API for computers to integrate "artificial artificial intelligence" directly into their processing by making requests of humans. Developers use the Amazon Mechanical Turk web service to submit tasks to the Amazon Mechanical Turk web site, approve completed tasks, and incorporate the answers into their software applications. To the application, the transaction looks very much like any remote procedure call: the application sends the request, and the service returns the results. Behind the scenes, a network of humans fuels this artificial artificial intelligence by coming to the web site, searching for and completing tasks, and receiving payment for their work.
Source: Amazon Mechanical Turk Help Page