While Japanese anime first appeared as early as 1917, it began to flourish in the 1960s with the spread of television throughout Japan. And over the years, its styles have changed.
This isn't new, and it's something Kotaku has touched on previously (here). However, below you can see images from 2ch that catalogue anime styles from the 1960s to this decade. They do an exhaustive job of illustrating the trends and evolutions in Japanese anime.