The Supreme Headquarters, International Espionage, Law-Enforcement Division, later known as Strategic Homeland Intervention, Enforcement and Logistics Division more commonly simply known as SHIELD, is a paramilitary organisation which deals with superpowered treats the world over.
Although some of these have been giant monsters or an assortment of supervilains, on of SHIELD reccuring foe is HYDRA a somewhat similarly structured organisation founded by ex-nazis for the purpose of world domination.
The uniforms seen below are based on the table of ranks published in the Official Handbook to the Marvel Universe v.1 # 10 (1983) It should be noted however that although the idea of colour coded accessories indicating ranks was used on and off from the 70s to the present days, the exact design of the uniforms and its component, even when used, tended to vary based on the illustrator so that the table below should be understood to be a generic representation only. in recent years, many depiction of SHIELD agents have had them wearing all the same colour of accessories (usualy white) over a blue-black skintight suit.
One last thing to note is that that "named" individuals (as opposed to background agents) often sported variation of the basic uniform or even a completely different suit so that their exact rank might not be indicated at all. For some reason, both in the Official Handbook to the Marvel Universe entry and in some, though not all, appearance in comics, Nick Fury sported a darker blue uniform then other agents.