In any story there is a good guy and a bad guy. That's the easy definition to the question posed to me by an acquaintance. More than that the protagonist is the main character of any story, the one the reader learns to sympathize with (most often). But protagonists can be depressing, flawed or troubled as well. They can be the person we love to hate. No matter what, they tend to be the lead character in our stories or plays. They can be the narrator or storyteller of the book as well.
On the other hand then, the antagonist is the person or thing that opposes the protagonist. They are the enemy of sorts, the one who puts up the barriers for our main character.
Just learning the simple terms like this helps a writer understand where his character is coming from. In my book, Emily Evans is my lead characer or protagonist. There are several antagonists. Her father, Denny, her mother Maureen and her own anger which all unite to keep her from giving in to her faith.
Hope this helps a bit! Have a good week!