Wikipedia
The Arrowverse is a shared fictional universe that is centered on television series airing on The CW, developed by Greg Berlanti, Marc Guggenheim, Andrew Kreisberg, Phil Klemmer and Geoff Johns, based on characters appearing in publications by DC Comics. The shared universe, much like the DC Universe in comic books or the cinematic DC Extended Universe, was established by crossing over common plot elements, settings, cast, and characters. The Arrowverse primarily stars Stephen Amell, Grant Gustin, Arthur Darvill, Megalyn Echikunwoke, Katie Cassidy, Wentworth Miller and John Barrowman.
The first television series was Arrow, based on the character Green Arrow, which debuted in October 2012. The universe was expanded with the series The Flash in October 2014, which is based on Barry Allen / Flash. The universe expanded further in August 2015 with the animated series Vixen on The CW Seed, and again with the live-action team up series Legends of Tomorrow in January 2016. The franchise has also crossed over with DC Comics television shows which air on other networks; Arrow crossed over with the short-lived NBC series Constantine in the former's fourth season, while the Flash appeared on an episode of Supergirl in its first season when it aired on CBS.
Many actors and characters have appeared across each of the series, with Miller, Barrowman and Cassidy signing deals to become series regulars across Arrow, The Flash and Legends of Tomorrow as Leonard Snart / Captain Cold, Malcolm Merlyn and Laurel Lance / Black Canary, respectively.