S1-5 is the Winchester Gospel - the main core story of the Winchesters, their role on earth and the apocalypse. S6-8 is the Purgatory Era - starting the whole thing with Castiel and Crowley with the souls from Purgatory, the Leviathans and Dean and Cas being in Purgatory ending with the trials, the angels falling and Castiel being human. S9-11 is the Heaven and Hell (or Angels and Demons) Era - starting with Sam being possessed by Gadreel, the Mark of Cain and Dean becoming a demon ending with the release of the Darkness and Cas letting Lucifer posses him to defeat Amara. S12-14 is the Alternative Universes Era - starting with Mary being back and introducing the BMOL, Cas dying, dealing with Jack, the AU and all types of ensemble cast type of characters (most ending up dead), ending with Chuck showing up to fuck things up. S15 is The End - it gives a new view to the whole story, where it all ties together and (hopefully) will have a satisfying ending.
S1-5 is the Winchester Gospel - the main core story of the Winchesters, their role on earth and the apocalypse. S6-8 is the Purgatory Era - starting the whole thing with Castiel and Crowley with the souls from Purgatory, the Leviathans and Dean and Cas being in Purgatory ending with the trials, the angels falling and Castiel being human. S9-11 is the Heaven and Hell (or Angels and Demons) Era - starting with Sam being possessed by Gadreel, the Mark of Cain and Dean becoming a demon ending with the release of the Darkness and Cas letting Lucifer posses him to defeat Amara. S12-14 is the Alternative Universes Era - starting with Mary being back and introducing the BMOL, Cas dying, dealing with Jack, the AU and all types of ensemble cast type of characters (most ending up dead), ending with Chuck showing up to fuck things up. S15 is The End - it gives a new view to the whole story, where it all ties together and (hopefully) will have a satisfying ending.