"What's past is prologue," wrote Shakespeare: In the beginning as the preface, from the Latin praefatio, meaning something said beforehand. It designated parts of the medieval church service, and Chaucer used it in the 14th century to mean a passage introducing a manuscript. Introduction, a synonym, dates from about 1600. Foreword, from the German Vorwort, appeared around 1900, along with the German university system.

For no good reason it was suddenly employed as a synonym for preface and introduction especially by Ph.D.s, whose degree was sometimes a preface to pedantry.

Oh, you wanted to know the difference. Today, according to The Chicago Manual of Style, the standard authority for such things, a preface is the author's brief discussion of his or her own work; someone else briefly comments on it in a foreword; and the author comments at length about the text in an introduction. The End.

Sources: THE OXFORD UNABRIDGED ENGLISH DICTIONARY, A DICTIONARY OF MODERN ENGLISH USAGE, & THE CHICAGO MANUAL OF STYLE)