527s
527s are tax-exempt organizations.
Many political organizations registered as 527s are called political action committees. Colloquially, only political organizations that evade the "express advocacy" legal definition are called 527s.
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Description
Section 527 is an IRS registration for tax-exempt organization.
History
Internal Revenue Code section 527 was produced in 1975. The legal standard at the time was that government can only regulate political parties, political candidates, and political organizations that "expressly advocate" for a candidate. As long as 527s avoid activities that fall under that legal definition, they are largely free of regulation.
The McCain-Feingold Act regulated any political speech leading up to an election, regardless of any "express advocacy" legal definition. This was upheld in !McConnell v. Federal Election Commission.
Between EMILY's List v. Federal Election Commission (2009), Citizens United v. Federal Election Commission (2010), and Speechnow.org v. Federal Election Commission (2010), the regulations on organizations that operate "independent expenditures" were thrown out.