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.
Yes, the characters depicted in the images are largely female, but many of them were aimed at female viewers. Some were aimed at both males and females, while others were created for decidedly male viewers.
Below, you can see how anime art has evolved, starting with Astro Boy's sister Uran and Princess Sapphire of Princess Knight. Both, of course, were created by Osamu Tezuka, the "Godfather of Anime". The timeline ends with more modern characters, such as Haruhi Suzumiya.
Note that some anime styles, such as Ghibli's or Sazae-san's, hasn't been as impacted as much by various trends.