S.H.I.E.L.D. (short for Strategic Homeland Intervention, Enforcement and Logistics Division) was a counter-espionage organization led by Nick Fury. They had their main headquarters in a flying aircraft carrier known as the Helicarrier.
Captain America: The Winter Soldier
In Captain America: The Winter Soldier, it was made known that SHIELD had been compromised all along by HYDRA sleeper agents. working within SHIELD. After foiling HYRDA's plan for world domination, SHIELD was disbanded.