Epic films are, in general, large-scale dramatic productions that contain adventures, normally set in times of war and conflict. The name is derived from the great stories and characters of the epic novel and originally the classic epic poem.
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.