Avengers is an Earth based science fiction which was released in 2012 and is directed by Joss Whedon.
After having been beaten by his brother Thor in the film of the same name, he has come back for revenge with help from the Skrulls. The Skrulls are a lizard like alien race that give the Avengers a run for their money.
When the Skrulls attack, Nick Fury of S.H.I.E.L.D. calls together the world's leading Marvel heros to fight the threat. S.H.I.E.L.D. is an organisation that is in the same vein as U.N.C.L.E., a group that fights threats. S.H.I.E.L.D. stands for Strategic Hazard Intervention Espionage Logistics Directorate. That was only from 1991, before that, it stood for Supreme Headquarters, International Espionage, Law-Enforcement Division
The Hulk, Captain America, Thor, Iron Man and Loki have all appeared in their own movies before appearing in this film. Natasha appeared as a secret agent of S.H.I.E.L.D. in the Iron Man films and Samuel's Nick Fury has guest appeared in other films. There has been no 'Hawkeye' films.
Nick Fury originally appeared as a white man in the comics but his race was later changed so it matched with Samuel L. Jackson. Nick appeared in a T.V. movie in 1998 starring David Hasslehoff but that has no connection to this Avengers film.