False dilemma is a logical fallacy in which a set of options are presented as if they are mutually exclusive or as if they're the only possible options, when in fact they are not.  It is used to give the impression that if one rejects another's argument, he must necessarily embrace an absurd or unappealing stance.
A false dichotomy is a special case of a false dilemma, in which only two options are presented.
Example of a false dichotomies include the arguments "if you oppose Barack Obama/Hillary Clinton, you're a racist/sexist!" They are false dichotomies because they ignore many possible reasons for opposing Obama/clinton.
Of course, not all dilemmas are false dilemmas.