S.H.I.E.L.D. stands for Strategic Homeland Intervention, Enforcement and Logistics Division. S.H.I.E.L.D. enforces peace in the world and makes sure that the Earth and the human race is in no immediate danger. S.H.I.E.L.D. was the group behind The Avengers Initiative. They created The Avengers to fight the battle that S.H.I.E.L.D. itself never could and to protect the world and human race from destruction.
Nick Fury is the head of S.H.I.E.L.D. and makes sure that everything happening in the world is under control. S.H.I.E.L.D. monitors any potential threats to the world to make sure they don't become threatening to the human race.
S.H.I.E.L.D. was created by Tony Stark's father Howard Stark. The organization was created to maintain world peace.