The central struggle between two characters or forces is the main conflict of a story or book. A conflict may be an internal struggle or external struggle.
An internal, or psychological, conflict takes place within a character's mind. Often, this is a conflict between right and wrong.
An external conflict involves a struggle between a character and an external force, such as another character, society or a force of nature. When an external conflict occurs between two characters, one character is the protagonist and the other character is the antagonist.
Other important elements in a story are setting, characters and resolution.