The antagonist is the main source of conflict for the main character(s) in your story. Often it might be the BAD GUY (school bully, like Greg in Speechless; the terrible Miss Trunchbull in Matilda etc.) That person may or may not be out to get the main character as such but may have goals that are opposite to those of the main character.

Sometimes that antagonist might be a group of people, like the Deatheaters in Harry Potter, or society as a whole, or even nature itself – the weather, such as a huge storm, an earthquake or a comet careering towards earth meaning imminent destruction of the world…