An antagonist is a character in a story who is presented as the main enemy and rival of the protagonist.
King Claudius, the antagonist, is married to Queen Gertrude in William Shakespeare's Hamlet.
A protagonist is the main character of a story. The protagonist makes key decisions that affect the plot, primarily influencing the story and propelling it forward, and is often the character who faces the most significant obstacles. If a story contains a subplot, or is a narrative made up of several stories, then each subplot may have its own protagonist.
Shakespeare's Hamlet, Prince of Denmark. William Morris Hunt, oil on canvas, c. 1864