A caramel apple is a whole apple coated in a layer of sweet, sticky caramel, traced to 1950s America, when confectioners began dipping apples in melted caramel as a novelty treat.
The preparation starts with a firm, crisp apple variety such as Granny Smith, Honeycrisp, or Fuji, threaded onto a stick. A caramel sauce is made by cooking sugar, often with butter, cream, or corn syrup, to the firm-ball stage (roughly 245-250°F/118-121°C), which yields a chewy rather than brittle coating once it hardens around the apple.
Caramel apples are strongly associated with fall harvest season, especially Halloween, and are often finished with chopped nuts, sprinkles, or chocolate drizzle. Outside the US, similar treats are called "toffee apples," though true toffee is cooked to a harder, glass-like hard-crack stage rather than the softer caramel stage.