What is the GRU of the Russian Armed Forces?

The Main Directorate of the General Staff of the Armed Forces (GRU) is the Russian military intelligence service. It was started in the Soviet Union, and its purpose is to gather important information and carry out cyber-attacks, destabilization moves, and covert and direct operations abroad.

What is the GRU of the Russian Armed Forces?
Official GRU flag. Source: Russian Ministry of Defense. Credit: Wikipedia