GRU is the English transliteration of the Russian acronym ГРУ, which stands for "Главное Разведывательное Управление" (Glavnoje Razvedyvatel'noje Upravlenije), meaning Main Intelligence Directorate. The full name is GRU GSh (GRU Generalnovo Shtaba (or "GenShtaba"), i.e. "GRU of the General Staff").
The GRU is Russia's largest intelligence agency. It deploys six times as many agents in foreign countries as the SVR which is the KGB intelligence successor. It also commanded 25,000 spetsnaz troops in 1997.
The current GRU Director is General Valentin Vladimirovich Korabelnikov.
The GRU was created in 1918 by Lenin, and given the task of handling all military intelligence. It operated residencies all over the world, along with the SIGINT (signals intelligence) station, in Lourdes, Cuba (22.995° N 82.464° W), and throughout the former Soviet bloc countries, especially in Lithuania, Latvia, and Estonia.
The GRU was well-known in the Soviet government for its fierce independence from rival power blocs, even the CPSU and KGB. At the time of the GRU's creation, Lenin infuriated the Cheka (predecessor of the KGB) by ordering it not to interfere with the GRU's operations. Nonetheless, the Cheka infiltrated the GRU in 1919. This planted the seed for a fierce rivalry between the two agencies, which were both engaged in espionage, and was even more intense than the rivalry between the FBI and CIA in America at that time.
The existence of the GRU was not publicized during the Soviet era. It became widely known in Russia, and the West outside the narrow confines of the intelligence community, during perestroika, in part thanks to the writings of "Viktor Suvorov" (Vladimir Rezun), a GRU agent who defected to Britain in 1978, and wrote about his experiences in the Soviet military and intelligence services. According to Suvorov, even the General Secretary of the Communist Party of the Soviet Union couldn't enter GRU headquarters without going through a security screening.
The GRU is still a very important part of the Russian Federation's intelligence services, especially since it was never split up like the KGB was. The KGB was dissolved after aiding a failed coup in 1991 against the then Soviet leader Mikhail Gorbachev. It has since been divided into the SVR (Foreign Intelligence Service) and the FSB (Federal Security Service).