Every storyline involves some kind of conflict. It is a struggle between two forces, but these forces can be either internal (feelings) or external (physical).
External conflict can exist between to characters, like the conflict that exists between a controlling father and youthful, mischievous son or the tension that occurs between a virtuous woman and a rogue of a man (and we know what can happen there).
External conflict can also be the conflict that occurs when a human encounters a physical challenge, like when a family is lost in a snowstorm.
Internal conflict exists when a character struggles with an ethical or emotional challenge. You can identify an internal conflict when you sense that a character is constantly asking himself or herself "Am I doing the right thing?" or "Should I speak out against this behavior?"
One story that contains a lot of internal conflict is The Red Badge of Courage. Henry evaluates his own self-worth constantly, as he observes, contemplates, and experiences fear, courage, bravery, and shame on the battlefield.
The internal conflict that a character experiences will usually represent a question about moral behavior within societies and among humankind. In The Red Badge of Courage, Henry experiences emotional turmoil because he is afraid of death (who isn't?) and he doesn't really want to kill others (who would?). As we read about Henry's experience, we can't help questioning the morality of war.