A preface would be almost like author's notes, you know how they researched the story or why a character is a certain way, what they went through to write it..etc
The prologue would be a dis-connected "pre-chapter" that may or may not make sense to the reader, but will connect eventually with the story.
So basically, both introduce you to the book- and don't quote me but are more of a trend in use (notice how many have prologues rather than prefaces during certain eras?), but you will almost certainly see more prefaces in non fiction, textbooks and the like.