Citizens United was a corporate legal case that became a U.S. Constitutional Law in 2010. The case, Citizens United v. Federal Election Commission, dealt with the regulation of campaign spending by organizations.
The United States Supreme Court decided by a close 5-4 vote that freedom of speech prohibited the government from restricting political expenditures by nonprofit corporations. Some of the basic tenets of the ruling were extended to for-profit corporations, labor unions, and other associations.
In a nutshell, the ruling gave corporations and unions the okay to spend unlimited amounts of money on ads and other political tools, in order to convince people to vote for or against a candidate, basically saying that spending is speech, and is therefore protected by the the United States Constitution.
Justice Stevens dissented, joined by Justices Ginsburg, Breyer, and Sotomayor.