Topics are sub-genres and/or themes of any kind (i.e. Romantic Comedy, Buddy, World War II, Sports) that enable you to more specifically describe the plot of the movie you are looking for. Topics, however, are not keywords. If you search for a topic like "shark", for example, only movies in which sharks play a significant roll will appear, such as 'Jaws'. But not every movie that has a shark in it will be listed.