The DC Extended Universe (or DCEU) is the name given to the films produced by Warner Brothers and centered around a shared universe of DC Comics superheroes. Warner Brothers has released many superhero films based on DC Comics characters, starting with Superman the Movie in 1978 and continuing through the various Batman films helmed by different directors and creative teams. The DCEU began with Zack Snyder’s Man of Steel (2013) and was an attempt to create a single, interconnected shared universe rather than independent, unrelated superhero films.
Man of Steel was greeted with mixed reviews, and the second DCEU movie, Batman v Superman: Dawn of Justice (2016) was a mess, and the third movie, Suicide Squad (also 2016) was at least a more entertaining mess. For a number of professional and tragic personal reasons, Zack Snyder left the DCEU before the completion of Justice League (2017) which was completed by Joss Whedon. It was a step up, but to date the best reviewed and performing DCEU movies have been the single-superhero films.
DCEU Wikipedia page
Movies discussed on The Ωmega Beam that are part of the DCEU: