Definition: In Episode IV: A New Hope, Obi-Wan Kenobi explains the Force to Luke as "an energy field created by all living things. It surrounds us, penetrates us, and binds the galaxy together." Jedi and other Force users access the Force with the help of midi-chlorians, microscopic organisms inside their cells.
The Force and the philosophies of its followers in the Star Wars universe bear resemblance to a number of real-world religions, including Hinduism (which includes a belief in a unifying Brahman energy, like the Force) and Zoroastrianism (which centers on the conflict between a good god, like the light side of the Force, and an evil god, like the dark side).
In-universe: Force-sensitivity varies with each individual, but some species are generally more Force-sensitive than others. For example, the Sith species, whose culture and philosophies would eventually evolve into an order of dark side users, was made up entirely of Force-sensitive beings. On the other hand, certain species, such as Hutts, lack Force-sensitivity and are resistant to Force powers.
Aside from the Jedi and Sith, there exist over fifty organizations and sects of Force users, each with varying philosophies on the nature of the Force and how to use it. By harnessing the power of the Force, Jedi and other Force users can gain extraordinary reflexes in battle, manipulate weak minds, heal, and even cheat death.