For example, 'The Fame' era vs. 'The Fame Monster' era:
vs
Music if 'fun' era's tends to be more upbeat with carefree lyrics, like Just Dance, and darker era's tend to have songs like Dance In The Dark and songs with deeper meanings, on top of the obvious change in looks/fashion...
Fun era's are easy to get into, just carefree and happy, but (most of the times) easy to forget, while the dark era's are timeless, after years the music still *****, because it isn't some throwback party pop, but it's deep and meaningful.
