The essence of Dispensationalism, then, is the distinction between Israel and the Church. This grows out of the dispensationalists' consistent employment of normal or plain interpretation, and it reflects an understanding of the basic purpose of God in all His dealings with mankind as that of glorifying Himself through salvation and other purposes as well. -- Charles C. Ryrie, Dispensationalism Today
I must note that I disagree with this appoach. A good, but brief, book that deals with Dispensationalism is Dispensationalism - Rightly Dividing the People of God? by Keith A. Mathison.