A show tune is a popular song originally written as part of the score of a “show” (or stage musical), especially if the piece in question has become a “standard,” more or less detached in most people's minds from the original context. Particular musicals that have yielded “show tunes” include.
Though show tunes vary in style, they do tend to share common characteristics—they usually fit the context of a story being told in the original musical, they are useful in enhancing and heightening choice moments.