Crowdsourcing is any sort of outsourcing that involves a large group of people actively participating in the project. Generally, this is the public, but a business might also utilise crowdsourcing techniques to develop an internal application.
Examples are large crowdsourcing towards a specific goal are all over, have a look at these few examples:
http://setiathome.berkeley.edu/https://github.com/
But there are many type of crowdsourcing, Crowdvoting, “Wisdom of the crowd", Crowdfunding,...