In creative writing terms, the antagonist refers to the character who opposes your protagonist, driving the plot forward. They are the serial killer in a gritty crime book, the evil Queen in the fairy tale, the power-crazy evil genius in James Bond.
If you're writing a good versus evil story then you will find it easy to identify your antagonist (hint: it's the big baddie), but what about different sorts of stories? Romances? Thoughtful family sagas? Literary musings on the meaning of life? Will you still have (or need) an antagonist, then?