From their very early conception in comic books, super-hero stories often take place in a shared universe. That is to say that they happen in the same settings as other works. Often times, these stories depict events that build upon things that have already happened in their world, fitting into the joint development of the storyline, characters, or setting of a larger overall project. This differs from crossovers where the works and characters are independent except for a single meeting.
If a show featured on this site takes place in a certain Universe, I have made an entry and attempted to watch every show in that Universe. As the shows are released I have created a Universal Watch Order (UWO) to help navigate the succession of the stories.