The term movie prop is short for movie property and refers to any object used by actors or set decorators in a movie. It generally refers to items used to define characters or to advance the story line. For example, a cane or a costume that helps define a character is a prop, while a tree or a chair is considered set decoration. Some movie props are highly collectible items.
A hero prop is any item intended to be held or used by one of the main actors in the film. These are often custom-made items. The term "hero" refers to the quality of the prop, with each one made well enough to be used in close-up shots. Actors occasionally drop and break props, so there may be more than one "hero" prop. An example of a "hero" prop would be the phasers held by Captain Kirk or Mister Spock in the "Star Trek" movies. Each one was made with close attention to detail.
Stunt props may be a gun or a chair smashed over an actor's head. They are often made of special material, such as breakaway glass, to protect the stunt actor. They're also made of material that can be easily destroyed, and are not subject to close-up shots. In a gun scene, for example, a prop gun for a hero will be made to look as authentic as possible, and rigged to fire blanks. Copies of the gun will be used as stunt props, but those guns won't be seen in close-ups and won't fire anything.
A background prop is not made to the same high quality as a hero prop. They are sometimes non-working versions of hero props, or props used in long-distance shots. Background props are often used when extras or background actors need to be seen with props. Background props may have minimum detailing or may appear rough close-up. For example, the background phasers used in the first "Star Trek" movie were vacuum-formed copies with no detail work.
Costumes, and in some cases clothing, may be considered a prop when it is custom-made for an actor or scene. Background costumes lack fine detail and may be made of lower quality materials. Stunt costumes may be created to be part of a stunt, or to hide mechanisms used in a stunt. Hero costumes are do have details for close-up scenes. Clothing may also be considered a prop if it's worn by one actor in an important scene. A famous costume prop would be Dorothy's ruby slippers from "The Wizard of Oz." A pair used in the film once sold at auction for $660,000.