An inter-dealer broker acts as an intermediary between other participants in capital markets (brokers and market makers) who wish to buy or sell large quantities without revealing their identities — i.e. offering a dark pool of liquidity.
Large inter-dealer brokers deal in huge volumes of securities (albeit at low commission rates). They have trading systems similar in scale and functionality to those used by stock exchanges, but inter-dealer brokers serve a more restricted market of professional investors.
Inter-dealer brokers may also be able to deal in securities and derivatives that are not traded on exchanges.