Midi-chlorian
Midi-chlorians (also spelled "midi-clorians" or "midichlorians") are mysterious organisms in the fictional Star Wars universe, first seen in the prequel trilogy. They are microscopic life-forms that reside within the cells of almost all living things and communicate with the Force. Midi-chlorians comprise a collective consciousness and intelligence, forming links between everything living and the Force. They are symbionts with all other living things; that is, without them, life could not exist. The Jedi have learned how to listen to and coordinate the midi-chlorians. If they quiet their minds, they can hear the midi-chlorians speaking to them, telling them the will of the Force. In order to be a Jedi or a Sith, one must have a high concentration of midi-chlorians in one's cells. This idea is somewhat similar to 17th-century philosopher Gottfried Leibniz's theory of monads -- infintesimal elementary particles that exist as something of an amalgam of matter and consciousness (termed 'apperception').
The word "midi-chlorian" appears to be a blend of "mitochondrion" and "chloroplast", two organelles found in real cells and thought to have evolved from bacteria as endosymbionts inside other cells, as purported in the endosymbiotic theory. Creator George Lucas has indeed stated that the midi-chlorians are based on the endosymbiotic theory, and it appears that in the story of Anakin Skywalker, he wanted to create a more modern "virgin birth" in the Star Wars saga that was as much based in science as in philosophy and religion, with the mythic "givers of life" being microscopic life-forms, rather than gods.
Criticism
Midi-chlorians were extremely controversial among fans when their existence was first revealed in Star Wars Episode I: The Phantom Menace. They were never mentioned in the original trilogy, and many fans believed they diluted the spiritual aspect of Star Wars by making the connection to the Force biological rather than mystical. However, it may have already been established in the original trilogy that the Force was genetic (through heredity) and passed down through families. Some argue that this was intentionally jarring; a way to imply that the Jedi Order of the time had become so bureaucratic that even one's potential in the ways of the Force was measured by a single statistic. Indeed, it would be only after the events of Episode III that one sees surviving Jedi explaining the Force in mythical terms, although Lucas insists that midi-chlorians were not mentioned in the original trilogy simply because he did not want to complicate the notion of Force too much at that time.
It has also been suggested that there are two schools of thought among the Republic's Jedi. On one hand, Qui-Gon Jinn's school would state that the Force is biologically caused by the existence of midichlorians, hence the reliance on Anakin's blood analysis to measure his ability with the Force. On the other hand, Yoda's school, which is the dominant school in the Jedi Council, would state that the Force is purely a mystical interconnection between all things — « Here, between you... me... the tree... the rock... everywhere! Yes, even between this land and that ship! » he teaches Luke in Ep. V — and that midichlorians are at most a consequence of the ability with the Force. Hence the reliance of the Council on true tests of Anakin's ability with the Force, even though Qui-Gon has certified the huge amount of midichlorians in his blood. We already know Qui-Gon, as his master Count Dooku, is a pariah among the Jedi, it wouldn't be surprising if he had a slightly different view of the Force than the dominant theory. And this would certainly explain why we didn't heard a word of the midichlorians in any of the five other Star Wars films.
Midi-chlorians and the Chosen One
It was foretold in an ancient prophecy that a chosen one would appear, imbued with an abundance of midi-chlorians, a being strong with the Force and destined to alter it forever. Anakin Skywalker was thought to be this being. His cells had the highest concentration of midi-chlorians ever before seen in a lifeform. He may not have even had a father; perhaps he was even conceived by midi-chlorians. Unfortunately, Supreme Chancellor Palpatine (alias Darth Sidious) greatly influenced Anakin, tending to the dark fires within him and converting him to the evil order of the Sith as Darth Vader. Anakin eventually fought a battle with his former friend and mentor, Obi-Wan Kenobi. During this duel, Anakin's body was ignited due to his proximity to a river of molten lava. Due to the tremendous amount of damage to his body, Anakin was fitted with a black suit that functioned as a life support system, completing his physical transformation into the fearsome character of the later episodes.
In Star Wars Episode III: Revenge of the Sith Supreme Chancellor Palpatine claims that a Sith Lord, Darth Plagueis, had the ability to use the Dark Side to influence midi-chlorians to create life, and so prevent people from dying. The truth of this claim is not established, although it has led some to theorize that Palpatine was behind the creation of Anakin Skywalker. However, this is never stated in the narrative and is pure speculation. Indeed, the official Star Wars website reveals that creating life and preventing death arise from the same ability, and since Palpatine openly admits that he cannot stop death, it seems highly unlikely that he has the ability to create life.
Plagueis's abilities create an interesting thematic development: the Sith attempt to gain immortality by unnaturally holding back death with the Dark Side and manipulating life, while the Jedi accept physical death as a natural part of life but learn how to retain their consciousness and corporeal spirits in the greater netherworld of the Force.