The social planets Saturn and Jupiter are contrasting energies, but they both deal with life at a societal level. While Jupiter is concerned with philosophy, meaning, and the ideal way to structure society, Saturn is concerned with making sure the village doesn't die of starvation this winter. As such, he's not nearly as cheerful a fellow, but you can learn a lot from him; you just might not enjoy the process.
Saturn rules Capricorn and is associated with the 10th house.
Saturn teaches us responsibility, self-discipline, determination, commitment, and self-reliance. Where we have Saturn Lessons, we often feel inadequate and awkward, like we just don't have what it takes to accomplish our goals. (Sometimes it's worse, more like self-loathing. Saturn can be quite painful.) Despite the difficulty, Saturn also gives us a determination to not give up. The combination, provided you don't completely resist the process, eventually produces maturity and mastery.
Saturn represents structures that protect and defend, which includes both social structures like governments, but also internal structures, like defense mechanisms.
When we are in the awkward-painful phase of doing our Saturn work (which may be most of our life, as Saturn work takes a long time), we will protect our pain with defensiveness. This is just a basic function of the human psyche: we want to keep people away from our bruises and wounds.
With Saturn, that defensiveness often takes the form of coldness, distance, criticism, rejection, or avoidance.
Saturn represents what we cannot change, and therefore points to where we must ourselves change.
We come into the time-space reality of form (which Saturn represents) for one purpose: to evolve our Soul. I think of it more as a training gym for the Soul than a classroom—and challenging us against our limits is a necessary part of the process of growth.
Saturn may be a difficult planet to learn from, but in the end, this is why we incarnated on Earth. If we just wanted to hang out in the ethereal world, we could have just stayed on the other side. Earth is a difficult school, and Saturn does not let anyone off easy. But if you take responsibility and do the work, you will attain that which you can only attain through genuine work and dedication, and it will feel worth it.
In traditional astrology, Saturn rules both Capricorn and Aquarius (which correspond to the 10th and 11th houses, respectively). Capricorn represents the collective traditions and rules of society, and Aquarius represents breaking free and finding your own truth.
While Jupiter and Saturn are a polarity between expansion and contraction, Saturn and Uranus are also a polarity between the status quo and revolution. Jupiter and Uranus both represent freedom, but in different ways—Jupiter is freedom to follow your inner compass, and Uranus is freedom from the known.
Saturn often plays the "Devil's Advocate" to other planets, including the Sun and Moon, which you can see in the sign polarities of Cancer-Capricorn and Leo-Aquarius.
Before the outer planets were discovered started in the late 1700s, Saturn was the edge of the solar system. It represented limitation, decay, and death (some of its traditional significations have moved to Pluto), as well as Father Time.
Sometimes Saturn is said to representing collective survival, structure, rules, and authority. Other times, Saturn is spoken of as an introverted planet that leads to self-criticism and fear, but ultimately fosters personal growth and maturation. So which is it?
These are both aspects of Saturn energy. In a psychological sense, Saturn helps us develop inner authority, self-discipline, personal responsibility, and to overcome our insecurity and master our fears. On a collective level, Saturn represents shared structures and institutions, like government and corporations and social norms. Saturn represents the principle of order and authority on a group level as well as a personal level. They work together in the sense that developing personal responsibility and maturity prepare you to participate constructively in society.
On a more esoteric level, Saturn represents how consciousness is conditioned. On an inner level, it is the conditioning we internalized as we grew up, which we have to face to transform. On a social level, it represents the status quo and consensus reality, which we have to grapple with to become individuals (Aquarius). On a societal level, Saturnian structures are disrupted and transformed by Uranus.