It's one of my favorites, you could call it a space opera as the main plot happens around various planets, space ships, distant travel, and different cultures.
It also has some interesting theory called psychohistory developed by a mathematician with the intent of predicting future events.
This was one of the greatest series, I thought. However, not many people thought the same. It is a shame. This series was in the first poll that I made. It may appear in the last one I make, too.
The foundation by Isaac Asimov