The Securities Investor Protection Corporation (SIPC) is a nonprofit corporation created by Congress to insure investors against losses caused by the failure of a brokerage firm.
Through SIPC, assets in your brokerage account are insured up to $500,000, including up to $100,000 in cash, but only against losses that result from the brokerage firm going bankrupt, not against market losses caused by trading decisions or other causes.
All brokers and dealers must register with the Securities and Exchange Commission (SEC) and are required to be SIPC members though they can lose their affiliation under certain circumstances. Clients of nonmember firms are not insured.