GRU is the English transliteration of the Russian acronym ГРУ, which stands for "Гла́вное Разве́дывательное Управле́ние" (Glavnoe Razvedyvatel'noe Upravlenie), meaning Main Intelligence Directorate. The full name is GRU GSh (GRU GenShtaba, i.e. "GRU of the General Staff"). 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 (signal intelligence) station, in Lourdes, Cuba (22.995 N, 82.464 W), and throughout the former Soviet bloc countries, especially in Lithuania, Latvia, and most other power centers in the Soviet Union, most famously the CPSU and KGB. At the time of the GRU's creation, Lenin ordered the Cheka (predecessor of the KGB) not to interfere with the GRU's operations. The rivalry between the GRU and KGB was even more intense than the rivalry between the FBI and CIA.
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 Communist Party general secretary couldn't enter GRU headquarters without going through a security screening.
The GRU still remains to this day 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 (1991) against then Soviet leader Mikhail Gorbachev. It has since been divided into the SVR (Foreign Intelligence Service) and the FSB (Federal Security Service).
According to the Federation of American Scientists: "...Though sometimes compared to the US Defense Intelligence Agency, [the GRU's] activities encompass those performed by nearly all joint US military intelligence agencies as well as other national US organizations. The GRU gathers HUMINT through military attaches and foreign agents. It also maintains significant signals intelligence (SIGINT) and imagery reconnaissance and satellite imagery (IMINT) capabilities."