Unmarked spoilers for Captain America: The Winter Soldier below, as it becomes crucial to the plot of the first season and the rest of the series.
Agents of S.H.I.E.L.D. (or Marvel's Agents of S.H.I.E.L.D.) is the first TV series set in the Marvel Cinematic Universe, focusing on the organization S.H.I.E.L.D. and taking place after The Avengers and Iron Man 3 chronologically. Premiering in September 2013 on ABC, it was created and executive produced by Joss Whedon, Jed Whedon and Maurissa Tancharoen (Joss Whedon also directed the pilot). It stars Ming-Na Wen, Elizabeth Henstridge, Iain De Caestecker, Brett Dalton, Chloe Bennet, and Clark Gregg as a very much alive Agent Phil Coulson, with later additions Adrianne Palicki, Nick Blood, Henry Simmons, and Luke Mitchell joining the main cast after its inaugural season.
The series began with the agents investigating and containing various supernatural and high-tech objects and individuals in order to keep the public safe, a task that has become more complicated in the aftermath of the Battle of New York blowing the lid off of their previous level of secrecy. The status quo was upended midway through the first season, when S.H.I.E.L.D. was dissolved after it was compromised by the terrorist group HYDRA. The series subsequently follows Coulson as his team struggles to rebuild the organization from its remaining loyal elements, even as they are being hunted by the U.S. Army, HYDRA, and other hostile forces of the Marvel Cinematic Universe, as Terrigen Mists empower latent Inhumans and threats both supernatural and extraterrestrial enter the fray.
While the previous Marvel Cinematic Universe films focused on Marvel Comics characters who already existed in the comics themselves, Agents of S.H.I.E.L.D. focuses primarily on characters original to the MCU, though there have been an increasing number of crossovers with comic and MCU characters, and several episodes have been direct follow-ups to the films. The reciprocity goes both ways, with events in the series seriously impacting the rest of the Marvel Cinematic Universe, while a comic book series with the Marvel 616 versions of the team ran from December 2014 to November 2015; a second one (bearing the same title as the show) launched as part of the All-New, All-Different Marvel initiative in January 2016.
A spinoff involving Lance Hunter and Mockingbird titled Marvel's Most Wanted was announced in April 2015 and later seemingly put on the backburner in favor of renewing Agents for a third season, and Agent Carter for a second season. Marvel's Most Wanted was cancelled officially alongside Agent Carter in May 2016. During December 2016, Marvel released a six-episode web series, Agents of S.H.I.E.L.D.: Slingshot, starring Natalia Cordova-Buckley as Elena "Yo-Yo" Rodriguez.
A sixth season was officially confirmed in May 2018, with plans for a reduced 13-episode run put into place instead of the usual 22. A mere six months later, ABC confirmed that the show will continue after that with a seventh season comprised of another 13 episodes.
There is a recap page for episodes.