DC U is a reinvention of the whole DC universe.
Clark Kent was raised on the stories of Superman, a character his father created to make his son happy. Years later, Clark finds a crashed alien space ship and meets Kal-El, who comes from a dead planet named Krypton. Kal-El is dying and asks Clark to fuse with him so that he can stay alive. Clark agrees and fuses with the alien, gaining superhuman abilities in the process. Clark decides to use these abilities to become the real life Superman.
Bruce Wayne was a soldier in the Vietnam War along side his friend Alfred Pennyworth. One day, Bruce was shot and killed by an unknown soldier. Bruce's rich parents spent a lot of their money to help their son. The research even continued after Bruce's parents had passed away. Eventually, years later, they brought Bruce back from the dead, not aged a day. However, Bruce wasn't happy about this, as even though the war was over, there was still horrible people in the world. After this, Bruce and the now elderly Alfred decide to take the law in to their own hands, as Bruce becomes the Batman.
Wonder Woman U
Many years ago there was a feared race known as the Amazons. They could defeat any enemy with no help needed. Though they realized they needed someone to protect their home of Themyscira, so they created Golems who would fight until they where destroyed. After many, many years, the Amazons either moved on, or died in combat. The only Golem left was named Diana, who decided to protect the rest of the world when she met the young pilot Steve Trevor.
The Flash U
Barry Allen and his friend Iris where at the launching of, what was called, a portal to another dimension. The portal opened up and it seems as if everything is going well, until Barry steps over the line and gets sucked in to the portal. Barry manages to escape using new super human speed he has due to the dimension. However, when he comes out, it is revealed what was two hours for him was two years in the normal world.
Green Lantern U
For thousand of years, the Guardians of the Universe have been unlocking living creatures who they deem worthy's full potential, creating "Green Lanterns". However, after all these years there has never been a human Green Lantern. That is until they find Mela Yin, a woman who has proven she has no fear what so ever. Because of this, she is made the very first human Green Lantern.
Arthur Curry is half Atlantean and the rightful heir to the Atlantean throne. He finds this out when he is visited by a group of Atlantean soldiers and his own half-brother, Orm, who reveals Curry's real name to be Orin. He agrees to come with them, but only if he still may visit the surface, as it is his home. They agree on this, and Orin is taken and crowned the Aquaman, the ruler of Atlantis.
Justice League U
The Justice League are a group of the world's greatest heroes. The League where formed to take down threats that none of them could ever take on alone.