A preface is an introduction to a book written by the author of the book. An introductory essay written by a different person is a foreword and precedes an author's preface. The preface often closes with acknowledgements of those who assisted in the project.
A preface generally covers the story of how the book came into being, or how the idea for the book was developed; this is often followed by thanks and acknowledgments to people who were helpful to the author during the time of writing.
A preface is usually signed (and the date and place of writing often follow the typeset signature); a foreword by another person is always signed. Information essential to the main text is generally placed in a set of explanatory notes, or perhaps in an "Introduction" that may be paginated with Arabic numerals, rather than in the preface. The term preface can also mean any preliminary or introductory statement. It is sometimes abbreviated pref.
Similarly, a prologue is typically an introduction to a novel, fitting in with the genre and storyline of the main text, rather than a section in the author's voice.
Preface comes from the Latin, meaning either "spoken before" (prae + fatia) or "made before" (prae + factum). While the former source of the word could have preface meaning the same as prologue, the latter strongly implies an introduction written before the body of the book. With this meaning of stated intention, British publishing up to at least the middle of the twentieth century distinguished between preface and introduction.