H.A.M.M.E.R. logoBecause of the 2012 Avengers movie, you may know about the Marvel Comics, global-security group S.H.I.E.L.D. However, there are several related groups.
Altered-Reality Monitoring and Operational Response
A.R.M.O.R. is an agency that researchs incursion with Earth-616 (the main Marvel Comics reality) with other realities. During Dark Reign, A.R.M.O.R. worked for H.A.M.M.E.R., but avoided being absorbed into it since Lyra (one of their members) found incriminating evidence against Norman Osborn. Their base is an underground banker.
S.H.I.E.L.D. was dissolved after a Skrull invasion. H.A.M.M.E.R. was created to replaced it. H.A.M.M.E.R. was controlled by the villain Harry Osborn. He fired anyone loyal to Iron Man, Captain America or Nick Fury. The organization was dissolved when Norman and his version of the Avengers illegally invaded Asgard. What the name stands for is unknown.
Special Tactical Response for International Key Emergencies
S.T.R.I.K.E. is a British security team similar to S.H.I.E.L.D. (despite the two being officially unrelated). Criminals infiltrated and took control over S.T.R.I.K.E. This led to S.T.R.I.K.E.'s disbanding.
European Monitoring Investigation and Enforcement Division
Unlike S.T.R.I.K.E., EuroM.I.N.D. was an European subdivision of S.H.I.E.L.D. However, Supreme Headquarters Allied Powers Europe (S.H.A.P.E.) took control of EuroM.I.N.D. François Borillon is the director. Originally, EuroM.I.N.D. had two subgroups: the scientist group Eurolab and the combatant group Task Force. However, the two groups merged into Euroforce.
Sentient World Observation and Response Department
S.W.O.R.D. is an agency that stops extraterrestrial threats. While it works with S.H.I.E.L.D., its connection to S.H.I.E.L.D. is unknown. Their base is a space station called the Peak. It should be noted there is similar group called Starcore.