Named after Mrs. Malaprop, a verbally inept character in Richard Sheridan's play The Rivals (1775), a malapropism is the confusion of two words that sound alike. Think of the albacore (instead of albatross) around Tony Soprano's neck, or Archie Bunker's reference to the Salivation Army.
Malapropisms share three traits: (1) the malapropism is always a real word, (2) it's usually unrelated in meaning to the intended word, and (3) both the error and the intended word are phonologically similar.
Also, as long as you're not the one who makes the verbal slip, a malapropism is occasionally funny. . . .
For the complete article (revised and expanded), see What Is a Malapropism? Decapitated Coffee and the Salivation Army.