If you're watching a Marvel movie, chances are that movie is happening in the same plane of existence as any other Marvel movie you may watch. This hasn't been the official case for the DC universe but they have recently come out to say that, from here out, that a shared DC film universe would be the standard.
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The Mary Sue points out some very valid information that was tucked into the coming movies announcement that supports this ideal. There are ten feature films coming up from DC which include two Justice League films as well as a new Wonder Woman, and also a standalone Flash and Cyborg movie.
These films will all be taking place in the same universe. It looks like the company is handling the Justice League films in much the same way that The Avengers is handled by Marvel. From the release it appears that the films focusing on single characters will be shown in conjunction with the massive team films, giving certain figures a chance to have better backstories. Which is a tactic that has worked out rather well for Marvel.
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DC hasn't seen as much success with its films as its competition Marvel has over the past few years and this new tactic may help them gain viewers. If nothing else, their movie line up has certainly excited long time DC fans as they will finally see more than just Superman and Batman on the big screen.