The Shadow Broker is an individual or organization which trades in information, always selling to the highest bidder. The Shadow Broker appears to be highly competent at its trade: all secrets that are bought and sold never allow one customer of the Broker to gain a significant advantage, forcing the customers to continue trading information to avoid becoming disadvantaged, allowing the Broker to remain in business.
The Shadow Broker's identity is unknown, since the Broker always operates through an agent. Barla_Vonrefers to the Broker as "he" for convenience's sake, but tells Commander_Shepardthat he believes the Shadow Broker is a group of individuals: it does not seem possible for a single individual to monitor all of the information and have such a wide sphere of influence.