Conflict is a struggle between two opposing forces, typically falling into the following categories: external and internal. Human against human, human against technology, and human against nature typically comprise external conflicts, and human against herself/himself make up internal conflict.
The conflict in any story is usually resolved at the climax. Resolution often involes a pivotal change in the the protagonist and/or antagonist (those opposing forces).
If you have problems identifying the conflict, ask yourself who or what is the biggest problem for the main character.
Another way to define conflict in a story is to look for the question that a story's plot attempts to answer. Conflict is the question or set of questions that will be resolved at the end of the story. It is, almost by default then, also the story's driving force.
For instance, in a detective story there will usually be a mystery to be solved. A murderer will need to be identified or a missing person will have to be found.
The question of "who did it" or "where is this person hiding/being kept" constitutes a large part of the story's conflict. This is true because it is this question that drives the story along and that keeps the reader turning the pages.
Even non-genre fiction like Moby Dick can be seen in this way. Despite the book's complexity and variability there is one question at its core - Will they get the whale or not?
Heart of Darkness and As I Lay Dying are another two examples of complex novels which generate momentum from a single, rather simple question that stands at the core of the text. d