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.

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, which followed the light side of the Force; and the Sith, who followed the dark side of the Force. When there was great loss of life, Force-sensitives could sense a disturbance in the Force.

Usage of the Force granted a number of useful powers, such as the ability tosense impending attacks, use the Physical Force to psychically push objects, influence the thoughts of others, known as the "mind trick," and even use the Unifying Force to see the future or maintain one's consciousness after death. Those who used the dark side could choke their opponents or 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 even 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, may have eventually changed 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 Antonio Motti, seeing Darth Vader as being nothing more than a sorcerer and 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. Droids, however, being mechanical creations, and thus possessing no true life, could never sense the Force.

History
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 committment 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 and later lost: in order to grow in power and eventually destroy the Jedi, the Sith begun to experiment with sorcery and rituals to cast powerful illusions and break Jedi so that they would fall to the dark side. 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 until the days of the Clone Wars on Moraband.

The wars between the Sith and the Jedi raged for centuries and several world of the galaxy were broken by their conflict, such as Moraband which the Sith were eventually forced to abandon. The tombs of the ancient Sith Lords on the world, in the Valley of the Dark Lords, were also marked with the darkness of the deeds of those who had been buried there. In addition to their experiments with sorcery, the Sith also took to building superweapons of power sufficient to destroy an entire world, events which would cause great disturbances in the Force. They used giant kyber crystals to power these weapons as the crystals could duplicate the size of a blaster ray. Both the Sith and the Jedi also armed themselves with their legendary weapons, the lightsaber, which were also powered by kyber crystals. As such, gaining control of the source of the kyber crystals (which were also powerful in the Force allowing them to identify their wielder and choose him in the case of the Jedi) was vital to both sides of the conflict.

Eventually, in the waning days of the Old Republic, Sith infighting had practically destroyed the practicioners of the dark side and the Jedi finally emerged victorious on their foes. A new galactic government, the Galactic Republic, emerged from the ruins of the previous regime and, with the Jedi as its guardians and advisors, 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: instead of several, there would now be only two Sith Lords; a Master, who possessed the power, and an apprentice, who craved it. As such, the infightings of the past would no longer occur and, from the shadows, 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 as well as his reformation of the Sith, 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.