Founded in Dublin, Ireland in 2001, Intrade gained widespread media attention as the world's leading prediction market during the 2008 and 2012 US election cycles for its accuracy predicting the outcome of those elections. Intrade closed its proprietary trading platform in early 2013.
Prediction Markets aggregate individual predictions; expressed as "prices", from trades made on an open exchange with "virtual" shares. By trading virtual shares about any yes or no question in real-time, and surfacing the price as a data point - Prediction Markets prices represent the probability of an outcome happening in the real world.
Because trading is incentivized by real money, Prediction Market forecasting is almost always more accurate than traditional forecasting methodologies.
In the simplest terms, Prediction Markets collect and aggregate the "Wisdom of the Crowd".