The Force

"Well, the Force is what gives a Jedi his power. It's an energy field created by all living things. It surrounds us and penetrates us; it binds the galaxy together."

- Obi-Wan Kenobi

The Force was an energy field that connected all living things in the galaxy. The power of the Force could be used by individuals who were sensitive to it, a power that was tapped through the midi-chlorians. The Force had many alternate names. The Lasats, for example, used to call it the Ashla, and described it as "the spirit of the galaxy."

Description
"For my ally is the Force, and a powerful ally it is. Life creates it, makes it grow. Its energy surrounds us and binds us."

- Jedi Master Yoda describing the Force to Luke Skywalker.

The Living Force represented the energies of all living things, and those energies were fed into the Cosmic Force that bound the galaxy together and communicated to individuals through the midi-chlorians. The two main practitioners of the Force's power were the Jedi Order and the Sith.

Usage of the Force granted a number of useful powers, such as the ability to sense impending attacks; to push and lift physical objects; influence the thoughts of others, known as the "Jedi mind trick"; and even see the future or maintain one's consciousness after death. Dark side users strong with the Force could summon lightning from their fingertips. Jedi taught younglings that the Force could be used for many purposes, including protection, persuasion, wisdom, the manipulation of matter and the performance of great physical feats.

Certain individuals that were not Force-sensitive sometimes doubted its very existence. For example, Han Solo once stated, when asked by Luke Skywalker if he believed in the Force, that "I've never seen anything to make me believe that there's one all-powerful Force controlling everything." Solo's adventures with Skywalker, however, did eventually change his opinion. This was also the case of Imperial officers during the later days of the Empire's reign: this led to several officers, such as Conan Antonio Motti, seeing Darth Vader as being nothing more than a sorcerer and the ways of the Force as an "ancient religion." However, higher-ranking officers who had been around during the Clone Wars, such as Grand Moff Wilhuff Tarkin, knew that the power of the Force was real; they simply believed that the Jedi were extinct and that Vader was the last who could use their power.

Though only certain individuals were Force-sensitive, the Force resided within all living things and could be extracted. Because of this, Force-sensitives could feel a "disturbance in the Force" whenever there was a great loss of life. Droids, however, being mechanical creations, and thus possessing no midi-chlorians, could never sense the Force.

Certain locations in the galaxy had strong connections to the Force, such as the cave Luke Skywalker entered during his training on Dagobah. There were also entire planets that had strong connections to the Force, these include the Force planet, Mortis and Moraband.

The ability to use the Force could run in a family line. The siblings Leia Organa and Luke Skywalker, children of the powerful Darth Vader, were both Force-sensitive. Organa would later bear a Force-sensitive child, Ben, with Han Solo, who was not Force-sensitive. Despite this, the Jedi policy of recruiting Padawan younglings at a young age and not having families typically meant in the time of the Galactic Republic that most individuals strong in the Force were born to those who did not have power in the Force. It was possible for siblings born to non-Force-sensitive parents to not share a talent for Force sensitivity. Such was the case with Zare Leonis and Dhara Leonis. While Dhara from an early age could always sense her brother's presence, Zare never had this talent, or indeed, any special abilities that could be attributed to the Force. Dhara's Force-sensitivity made her a target for the Grand Inquisitor of the Galactic Empire, whereas Zare was able to enter the Empire's ranks undetected in order to seek out the truth behind her disappearance.

The Hundred-Year Darkness
"Once we were brothers in the Force. But from the Hundred-Year Darkness were born the Sith."

- Unidentified Jedi

Thousands of years before the Clone Wars, the Jedi Order and the individuals who became the Sith were considered brothers in the Force. This unity fractured, however, during the period of time known as the Hundred-Year Darkness, when a rogue Jedi came to the understanding that the true power of the Force lay not through contemplation and passivity, and that only by tapping its dark side could its true potential be gained. The Jedi High Council at the time balked at this new direction and the Dark Jedi was outcast, but he eventually gained followers to his new order, which resulted in the formation of the Sith Order.

As a result of the schism between the Jedi and the Sith, the Force came to be seen as divided between their two philosophies: the Jedi's selfless commitment to serving and defending came to be regarded as the light side of the Force while the Sith's use of emotion and violence became known as the dark side of the Force.

The Force at war
As the schism between Jedi and Sith grew, their open hostility eventually turned to open war for both control of the galaxy and to impose their own philosophy with regards to the Force. During the days of the Old Republic, Sith and Jedi fought countless wars to destroy one another. Great battles were fought between the two warring orders and various ways of using the Force were discovered but later lost. The Sith also took to sacrificing Jedi in ancient chambers on their homeworld of Moraband, places which would remain forever tainted with the darkness of the events that happened in there. The Sith also used these arts to create monsters, such as Sith snakes, which remained on Moraband until the days of the Clone Wars.

The wars between the Sith and the Jedi raged for centuries and several worlds of the galaxy were broken by their conflict, such as Moraband, which the Sith were eventually forced to abandon. The Sith tombs in the Valley of the Dark Lords were also marked with the darkness of the deeds of those who had been buried there. Both the Jedi and the Sith used lightsabers as their main weapons. These devices were powered by kyber crystals, special crystals with a strong connection to the Force. In addition to their study of the dark side, the Sith also took to building superweapons of power sufficient to destroy an entire planet. They would sometimes use giant kyber crystals to power these weapons. . As such, gaining control of the source of the kyber crystals (which had a strong connection the Force) was vital to both sides of the conflict.

In the time before the Empire took over, the Jedi started using midi-chlorians to measure potential in the Force. A Jedi Knight, Qui-Gonn Jin, discovered a boy named Anakin Skywalker on Tatooine that was more powerful in the Force than any known Jedi. The boy became a powerful Jedi, then a powerful Sith Lord, and eventually fulfilled his destiny and brought balance to the Force, as the prophecy said.

Eventually, in the waning days of the Old Republic, Sith infighting had practically destroyed themselves and the Jedi finally emerged victorious. A new galactic government, the Galactic Republic, emerged from the ruins of the previous regime and, with the Jedi as its guardians and advisers, they expanded throughout the galaxy and begun an age of peace and prosperity which would last for a thousand years.

However, despite the Jedi's victory and the fact that the Force was now dominated by their light side philosophy, the Sith had secretly survived. The sole surviving Sith Lord was named Darth Bane and, realizing that the Sith's defeat had come from their infighting and greed, he created a new guiding principle for their Order known as the Rule of Two: instead of several, there would now only be two Sith Lords; a Master, who possessed the power, and an apprentice, who craved it. As such, the infighting of the past would no longer occur and the Sith could regain their power in secret until the time was right for them to have their revenge on the Jedi and reclaim the rule of the galaxy. Although the Jedi eventually discovered that Darth Bane had survived, they believed that, upon his death, the Sith were truly extinct.

Behind the scenes
The leitmotif of the Force is called Binary Sunset. In  The Clone Wars, a deep rumble was typically used as the sound effect for the dark side of the Force. In Star Wars Rebels, a high-pitched tone likened to a wind is used as the sound effect for the light side of the Force.