In books and other works, a subtitle is an explanatory or alternate title. As an example, Mary Shelley gave her most famous novel the title Frankenstein; or, the Modern Prometheus; by using the subtitle "the Modern Prometheus", she references the Greek Titan as a hint of the novel's themes.
In English, subtitles were traditionally denoted and separated from the title proper by the conjunction "or", perhaps hinting at their function as an alternate title. A more modern usage is to simply separate the subtitle by punctuation, making the subtitle more of a continuation or sub-element of the title proper.
Subtitles for plays were fashionable in the Elizabethan era; William Shakespeare parodied this vogue by giving Twelfth Night his only subtitle, the deliberately uninformative What You Will, implying that the subtitle can be whatever the audience wants it to be. In printing, subtitles often appear below the title in a less prominent typeface or following the title after a colon.
Some modern publishers choose to forgo subtitles when republishing historical works, such as Shelley's famous story, which is often now sold simply as Frankenstein.
In library cataloging the subtitle does not include an alternate title which is defined as part of the title proper; e.g. One Good Turn: A Natural History of the Screwdriver and the Screw would be filed as "One Good Turn" (title proper) "A Natural History of the Screwdriver and the Screw" (subtitle), while Twelfth Night, or What You Will would be filed as "Twelfth Night, or What You Will" (title proper).
Film and other media
In film, this has been used in films such as Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb and Birdman or (The Unexpected Virtue of Ignorance).
Subtitles are also used to distinguish different installments in a series, instead of or in addition to a number, such as: Pirates of the Caribbean: Dead Man's Chest, the second in the Pirates of the Caribbean series; Mario Kart: Super Circuit, the third in the Mario Kart series; and Star Trek II: The Wrath of Khan, the second in the Star Trek series.