Re: Hels/Steve R
Strategic Homeland Intervention, Enforcement and Logistics Division. A counterterrorism and intelligence agency, led by Nick Fury then Phil Coulson. Originally founded by Howard Stark, who recruited Agent Peggy Carter from the SSR to run it with him. The SSR, of course, being responsible for the development of the super soldier serum that you took.
Post WWII, they were infiltrated by Arnim Zola, who rebuilt HYDRA inside of S.H.I.E.L.D. until such a time as they were ready to reveal themselves to the world. Which they did, with Project Insight and the three helicarriers. (Has that happened here yet? I've only seen the movies.)
Now it's hard to tell who's HYDRA and who's S.H.I.E.L.D. And in the movies, HYDRA was experimenting on people.