In traditional astrology Saturn rules Capricorn and Aquarius.
I love how Roy Gillett describes Saturn: "Saturn indicates where problems, restrictions, and barriers are likely in our lives and confronts us with the reality of impermanence. But Saturn presides over much more than failure and endings: as the planet of firm foundations, it can show us how to strengthen our endeavors and make them more sustainable".
Saturn is necessary for the human experience. Keywords for Saturn include discipline, responsibility, structure, pragmatism, and anxiety. I have also seen it represent the father figure in birth charts.
A well placed Saturn is reserved. An ill placed Saturn is mistrustful and jealous. Saturn is most at home in Capricorn, Aquarius, and Libra. It doesn't do as well in Cancer, Leo, and Aries.