Without revealing any spoilers concerning information beyond that which has been televised thusfar, can someone please answer how closely related the TV show Marvel's Agents of S.H.I.E.L.D. is to the comic books that the show is based off of?
Agents of S.H.I.E.L.D. was not originally based on any single comic. It takes its influences from a number of different comic continuities and the Marvel Cinematic Universe. In the canon comics, the Strategic Homeland Intervention, Enforcement and Logistics Division, was a peacekeeping, intelligence agency in the canon Marvel Earth-616 continuity.
S.H.I.E.L.D. was an organization well known in the canon Marvel Universe and had its fingers in any number of pies depending on who was writing it, what era it was being read in and what kinds of interactions they wanted the agency to have with superheroes.
For most of its history, it has been run by Nick Fury, the now (and former) director of the agency. Almost none of the characters created for the television show except for Nick Fury existed before the show. Some of them, such as Agents Coulson, may have come from the Marvel Cinematic Universe movie line.
There have been comics which featured the organization such as Nick Fury, Agent of S.H.I.E.L.D., Captain America who was an off-again, on-again agent/operative, appearances of the Black Widow in various Avengers, Iron Man and Daredevil related books. While extremely popular in the seventies and early eighties, the agency had taken a bit of a back burner until the recent Civil Wars, Superhero Registration series of events.
- S.H.I.E.L.D. also regained prominence in recent years in Marvel's Ultimates Universe Earth-1064, where S.H.I.E.L.D. was responsible for the creation of many of the super-heroes of that continuity and where Nick Fury was a Black man who resembled his counterpart in the Marvel Cinematic Universe, Samuel L. Jackson. This same agency is also the coordinator for metahumans in the Ultimate Spiderman Animated TV Series.
The Agents of S.H.I.E.L.D. television show has very little to do with the comics. Most of the characters in the show do not appear in the comics at all. Agent Coulson was created for the Marvel Cinematic Universe, and was only added as a character in the comics later. The agents who work for Coulson were created for the TV series. Only a handful of guest characters on the show, such as Maria Hill, Nick Fury, and Sif appear in the comics (and films). Each episode's mission is written for the show, and are only loosely based on events and characters from the comics.
The cinematic universe and the comic universe are not entirely related. They differ heavily on certain aspects. For instance in the comic universe the wikipedia states.
S.H.I.E.L.D. started off as a top secret international organization (Fury was unaware of them when he was in the CIA) with a Supreme International Council made up of top officials and minds from across the world, including Tony Stark.
Another difference would be the fact that a lot of the villains / organizations are unique to the series like the Clairvoyant and Centipede, although they tie in to stuff that actually occurs in the comic books.
You may have noticed often names are changed or secret identities used. Bobbie Morse is Mockingbird, Calvin is Mr Hyde - obviously Skye/Daisy is Quake. Absorbing Man and Deathlok, plus a hint at Graviton (even Blizzard possibly?) I've not gone into SHIELD comics that much, or Inhumans for that matter, but have seen some names I know.