American Horror Story is an American horror anthology series that began airing on the FX Network in October of 2011. The series was created by Brad Falchuk and Ryan Murphy and produced by 20th Century Fox Television. The series is unique from other such programs in that each season is it's own insular story-arc with a different cast of characters and actors. Each season of the show averages between 10-13 episodes.
Every season of American Horror Story falls under a subtitle relating to the storyline from that season. While each season was originally crafted as taking place in their own continuity, separate from that of other seasons, the show began slowly weaving connections between the seasons through various characters and references. By season seven, all seasons of the show could be connected to one another, and now inhabit a single continuity. Season one was titled "Murder House", and chronicled the lives of a family who moved into a haunted house. Season two took place in the 1960s and was called "Asylum", and took place at a mental institution. Season three was called "Coven" and involved the political intrigues of a coven of witches in New Orleans. Season four was "Freak Show", which showcased a carnival sideshow from Jupiter, Florida in the 1950s. Season five was "Hotel", which was set in the modern era and involved vampires and serial killers. Season six was dubbed "Roanoke" and was filmed in documentary style with cast members playing characters from the story, as well as re-enactment actors. Season seven was a political intrigue story called "Cult" and was the first season of the show to contain no supernatural elements. Season eight was "Apocalypse", which chronicled the aftermath of nuclear Armageddon.
Grammy-award winning pop star Lady Gaga was the first celebrity cast to appear on season five of American Horror Story. No other details relating to the show were released at that time, but Gaga revealed in a tweet that the show would sport the serial name, "Hotel". 
Starting with the fourth season, Freak Show, several characters began re-appearing in order to connect each universe to one another. They are as follows:
Sarah Paulson is the only actor to appear in all nine seasons. Evan Peters did not appear in 1984 though he still remains the only AHS veteran to be a main cast member in the eight seasons he appeared in. Lily Rabe and Frances Conroy have the second-most appearances with seven as Rabe did not appear in Cult and Conroy did not appear in Hotel.
On Thursday, July 19th2018, it was announced that the eighth season of American Horror Story would bear the subtitle "Apocalypse", with storylines branching off and interconnecting from the season one serial, "Murder House", and season three's "Coven". Season one in particular promises to have a strong connection as it saw the birth of the Antichrist. Showrunner Ryan Murphy released the title and artwork on Thursday evening at the San Diego Comic-Con. 
It was announced at the 2018 San Diego Comic-Con that actress Joan Collins would be joining the cast of American Horror Story for season eight.
It was announced in August2018 that AHS veteran Jessica Lange would be returning for season eight after her departure from the series following season four.
It was announced in April2019 that AHS veteran and mainstay Evan Peters would not return of season nine, making it the first season he will not be featured in. Sarah Paulson will also appear in a limited role, making it the first season since Murder House where she isn't a member of the main cast.