Crowdsourcing is a neologism, consisting of a portmanteau of "crowd" and "outsourcing," which denotes the process of outsourcing a task or project to a large undefined group of often anonymous volunteers, typically using the internet and so-called "Web 2.0" technologies such as social networking.
The three main advantages of "crowdsourcing" is that certain tasks can be accomplished (1) much more quickly, (2) at lower cost, and (3) often with better or more innovative results than if using traditional human resources such as full-time employees, subcontractors, free-lancers, or screened volunteers.
The lower costs come from the fact that crowdsourced workers are typically unpaid volunteers or at best minimally paid. The time savings come from the large numbers of workers used, while the higher quality comes from the fact that more brainpower drawn from a wider pool can bring more and better ideas to bear on a given problem.
Major disadvantages of crowdsourcing include the difficulty of ensuring quality control across all aspects of the project, difficulties in maintaining required levels of interest and commitment among workers over the long term, and difficulties maintaining or controlling the direction or vision of a project. It can also be very difficult to profit from crowdsourcing, as unpaid or minimally paid volunteers tend to react negatively if they learn that their hard work is being taken advantage of for personal gain.
*The* classic example of crowdsourcing is of course Wikipedia. Other famous examples include Berkeley's SETI@home project, which takes advantage of the unused processing power of tens of thousands of home computers to crunch numbers toward finding extraterrestrial life, and services such as Yahoo Answers or urbandictionary.com which draw upon vast pools of volunteers to uncover sources of specific knowledge.
The origin of the term "crowdsourcing" appears to be a 2006 article by Jeff Howe in Wired Magazine.