The word "monologue" derives from the Latin roots "mono," which means "single" or "alone" and "logue" which means "to speak." A monologue, then, is a speech by one person. In a play, a dramatic monologue occurs when a character talks to himself out loud when no other characters are either on stage or within hearing distance. An interior monologue, a stream of a character's thoughts or emotions, lends itself well to novels, where a narrator gives the reader access to what goes through a character's mind.
Novels, poetry and drama all contain famous monologues. The narrator in Robert Browning's poem "My Last Duchess" presents a monologue, in which his cold portrayal of his dead wife reveals him to be a rather horrifying character. William Faulkner's World War I novel "The Sound and the Fury" uses streams of consciousness to record the interior monologues of several main characters' experiences.
A soliloquy and a dramatic monologue are so similar, the two terms are often used interchangeably. A soliloquy is a monologue restricted to drama that actors speak only when alone, or when they believe they are alone. Because a play contains mostly dialogue and very little narration, a soliloquy is the only time an audience can access a character's mind. Soliloquies are generally thought to be genuine because when no one is listening, the character has no motivation to lie, whereas other times in a play a character's speech might not be trustworthy.
Shakespeare often used soliloquies. Perhaps the most famous soliloquy of all time is Hamlet's "To be, or not to be" speech, where he contemplates whether he should kill his father, and indeed, whether he should even continue living. In "Romeo and Juliet," Juliet gives a soliloquy --- "Romeo, Romeo, wherefore art thou Romeo?" --- not realizing that Romeo in fact overhears her.