Conflict: struggle a struggle or problem in a story causing tension.
In literature, every story from Hamlet to Harry Potter has some sort of conflict. Whether it is the internal struggle of a character or a great evil the protagonist must destroy, all stories have a conflict. The conflict is the main driving point of the story for with out a conflict there would be no story at all. If Hamlet's father never was murdered, Hamlet would have never had to avenge his death. If the Montagues and Capulets didn't have a bloody feud, Romeo and Juliet would have no problem loving each other. The conflict makes the story.
Here is a video that shows different types of conflicts in literature.
Here is a simple version of a conflict between two characters.