The Financial Industry Regulatory Authority (FINRA) describes itself as the largest independent regulator for all securities firms doing business in the United States. It was created in July of 2007 through the consolidation of the National Association of Securities Dealers and the enforcement arm of the New York Stock Exchange. FINRA operates the largest arbitration forum for the resolution of disputes between its member firms and their customers. Most of the agreements between brokerage firms and investors include mandatory arbitration provisions, which require that any dispute be arbitrated before a FINRA panel. Because of this, you can not sue your brokerage firm in most civil litigation forums and your only avenue to pursue a claim to recover losses is by filing a formal claim for FINRA arbitration.
For more information on the difference between a FINRA Complaint and FINRA Arbitration, watch this video from attorney Courtney Werning.