The struggle between opposing forces is conflict, and is usually the basis for the plot since that's what holds the audience's interest. 

An external conflict is one against an outside force, such as nature, a physical obstacle or another character.  An internal conflict is one that occurs within the character.

There are a few basic types of conflict:
   Man vs. Self
   Man vs. Man
   Man vs. Society
   Man vs. Nature
   Man vs. Supernatural
   Man vs. Machine