In most cases the conflict results when someone or something prevents the protagonist from achieving their goal.
External Conflict:The struggle between a character and an outside force. This can be another character, societal or community beliefs, the legal system and even forces of nature
Man vs Man: typifies the daily troubles we have with others. This type of conflict finds the main character in conflict with another character Example: Harry Potter vs Lord Vlodemort
Man vs Nature:one of the very first struggles we as a civilization faced. This conflict finds the main character in conflict with the forces of nature, which serve as the antagonist. Example: Joe vs The Volcano
Man vs Society: the main character in conflict with a larger group: a community, society, culture, etc… A character’s main struggle with mainstream beliefs or community values. Example: The Hungar Games