Open data is data that can be freely used, reused and redistributed by anyone - subject only, at most, to the requirement to attribute and sharealike.
Not only the data, but the code used to create government websites, portals, tools, and other online resources can provide further benefits, as valuable open data itself. Governments should employ open source solutions whenever possible to enable sharing and make the most out of these benefits. The Consumer Finance Protection Bureau (CFPB) began publishing open code on the social code site GitHub in 2012, citing that doing so helped them fulfill the mission of their agency and facilitated their technical work. (More information is available in the announcement blogpost on the CFPB’s website.)
The New York Times called this discovery of uses for public data in both government and private markets the "Moneyball" revolution:
The story is similar in fields as varied as science and sports, advertising and public health — a drift toward data-driven discovery and decision-making.