Introduction, prologue, prelude, preface, foreword, exordium, preamble are comparable when denoting something that serves as a preliminary or as an antecedent to an extended treatment, development, discussion, or presentation (as in an exposition, a dramatic or musical work, or a poem).
In their extended senses many of these terms are interchangeable, but in the special or technical senses in which they are here chiefly considered they tend to be mutually exclusive.
Introduction, the ordinary term of this group, and the comprehensive one, specifically applies to that part of a work (as a discourse, treatise, play, or musical composition) which prepares the reader or auditor for the body of the work, especially by giving him material necessary for his understanding of what follows.
Prologue applies specifically to the initial and distinct part of a poetic or dramatic work which may serve the purposes of an introduction (as by describing the characters or expounding the situations in which they find themselves) or which may be a discourse preceding the opening of a play, by a character of the play or by an actor who serves as a mouthpiece for the author, and giving a hint of the author's purposes or methods or attempting to attract the auditors' attention to or interest in the play to come. In its extended use, therefore, prologue often suggests an action or an event that sets the stage or paves the way for a series of exploits, achievements, or significant events.
Prelude applies in its general sense to something (as a series of events, actions, or natural phenomena) which constitute figuratively a short play or performance and serve as a sign or indication of or a preparation for what is to follow. As a technical term in music prelude applies sometimes to an opening voluntary in a religious service but more often, and more specifically, to an introductory piece forming a section or a movement, especially of a fugue or a suite but sometimes of an oratorio or of an opera, and serving usually to introduce the theme or chief subject of the work. In this sense prelude applies sometimes to musical, or occasionally other, works which have something of the character of an introductory section or movement but are so constructed that they have intrinsic and independent value.
In ecclesiastical use preface applies to the prayer of exhortation to thanksgiving and of divine praise which opens the important part of a solemn Eucharistic service where the consecration of the bread and wine occurs. In its more common general sense preface applies specifically to a short discourse which is distinct from the literary work (as a treatise, a novel, a poem or collection of poems) which follows, is written usually by the author but sometimes by an editor or a friend, and has for its main purpose either to put the reader into the right frame of mind for the understanding or appreciation of the work he is about to read or to supply him with information that may be necessary to his proper understanding or use of it.
When, however, a work is preceded by both a preface and an introduction, preface is usually applied to the introductory discourse written, and often also signed, by the author or editor, and introduction to the one which is definitely informative rather than personal in its character and usually carries no signature.
In extended use preface may apply to something which serves as an introduction or prelude (as an introductory work on or a more or less tentative treatment of a subject) or to an act or speech, or series of acts or speeches, which has no other purpose than to prepare the way for what is to follow.
Foreword when used in place of preface in reference to front matter of a book may suggest simplicity and brevity of treatment and more often than not applies to material prepared by someone other than the author.
Exordium, a technical term of rhetoric, applies to a formal beginning, especially of an oration but sometimes of a written exposition or argument, in which the speaker or writer makes an approach to his subject by remarks intended to awaken the interest of his auditors or readers and to pave the way for their understanding of what he is to say or for their acceptance of his conclusions.
Preamble applies to a formal introduction, often only an introductory paragraph (as in a statute, a constitution, a treaty, a deed, or a set of resolutions) which states the grounds, purposes, or guiding principles of what follows. It is sometimes used as a designation of a long monotonous preface.