SIPC stands for the Securities Investor Protection Corporation, which is a non-profit, non-government, membership organization that consists of broker-dealers. Almost all broker-dealers that are registered with the SEC are members of this corporation.
The main purpose of the SIPC is to recover securities and funds for investors if a broker-dealer becomes insolvent or bankrupt. SIPC coverage also protects investors in unauthorized trading cases.
Broker-dealers that belong to the SIPC are required to display an official sign reflecting their membership in the organization. If you are not sure if your broker is a member, you can research the SIPC member database.