Darby is noted in the theological world as the father of "
dispensationalism," later made popular in the United States by
Cyrus Scofield's
Scofield Reference Bible. He originated the "secret rapture" theory wherein Christ will snatch away his true believers from this world without warning. Dispensationalist beliefs about the fate of the Jews and the re-establishment of the Kingdom of Israel put dispensationalists at the forefront of
Christian Zionism.
Here a bio:
http://www.johndarby.org/beginning/index.html
This site also shines a "light" on Darby and is a very extensive study on Darbyism:

A must read.
http://www.graceonlinelibrary.org/articles/subcats.asp?id=9|21
C. I. Scofield by
William E. Cox
The father of dispensationalism, Darby, as well as his teachings, probably would be unheard of today were it not for his devoted follower, Scofield. The writer became increasingly aware of this fact as he did research for this book. Darby's books are gathering dust on the shelves of the comparatively few libraries stocking them. Information concerning him is scarce indeed.
Is the Pretribulation Rapture Biblical? by
Brian Schwertley
Although the pretribulation rapture doctrine is very popular and is even considered so crucial to Christianity that it is made a test of a person’s orthodoxy in some denominations, Bible colleges and seminaries, the exegetical and theological arguments used by its advocates are all classic cases of forcing one’s theological presuppositions onto particular texts (eisegesis). The purpose of this brief study is to show that the pretribulation rapture theory is not plainly taught or directly stated in any place in Scripture, cannot be deduced from biblical teaching, contradicts the general teaching of the Bible regarding Christ’s second coming and was never taught in any branch of the church prior to 1830.
http://www.kingdombiblestudies.org/Looking/Looking9.htm
LOOKING FOR HIS APPEARING
This distorted conception of the return of Christ, and its purpose, caused the Church of England scholars to translate the Greek word PAROUSIA by our English word "coming," when its real meaning is "presence." In their twisted theology there was simply no way in which Christ could be present in the earth for more than a few hours. He was to return, and rapture His people almost immediately away to heaven, so they could not conceive of His coming meaning His PRESENCE. The word PAROUSIA is used twice in the New Testament when it does not apply to Jesus, but to the apostle Paul. These uses of the word establish its real meaning. Paul expresses others' appraisal of him, saying, "His letters, say they, are weighty and powerful; but his bodily presence (PAROUSIA) is weak" (II Cor. 10:10). And again, writing to the saints at Philippi, Paul says, "Wherefore, my beloved, as ye have always obeyed, not as in my presence (PAROUSIA) only, but now much more in my absence, work out your own salvation with fear and trembling" (Phil. 2:12).
It can be seen at once that the use of the word "coming" to translate the Greek word PAROUSIA in the foregoing texts would have given an entirely wrong meaning to Paul's words. In these passages it is the PRESENCE of a human being that is involved, so we understand what Paul means. He is not discussing his "coming" or "arrival," his "journey" to them, but his actual presence in their midst. So we need only to move from the understandable into the realm in which we do not fully understand in order to realize what the prophecies mean which discuss the PAROUSIA of our Lord; that meaning being His ACTUAL PRESENCE, not His "coming."