External conflict comes from without.  It is an event, a life change, a person, something or someone to present discord in the lives of the protagonists.  Use it as a catalyst for character growth.  It is, according to The Handbook of Literature, what provides interest and suspense in plot, incorporating not only the protagonist's struggle against something, but also a motivation for the struggle and the eventual means to win it. 

(Also see:  "Internal Conflict")

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