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:
But there are many type of crowdsourcing, Crowdvoting, “Wisdom of the crowd", Crowdfunding, Microwork, Inducement prize contests and Implicit crowdsourcing.
Lately the most popular of these types have been crowdfunding where startup companies can market their planned projects and ask for financial assistance from the public, for which they will receive special non-recognition from the company.
You can find some really great examples here: