In pool/billiards, where the term is most commonly used, it implies a shot that is rarely used or seen, and the usefulness of which is dubious and usually employed for amusement versus actual play. There are many trick shots in pool that are useful in tournament play, but many are just for show.
Also, along the same lines, trick shot can also be used as a personal descriptor for a player as someone who is skilled in the above actions, as in "She's a real trick shot". | 
