From the Wikipedia

The informal fallacy of false dilemma—also known as false choice, false dichotomy, falsified dilemma, fallacy of the excluded middle, black and white thinking, false correlative, either/or fallacy, and bifurcation—involves a situation in which two alternative statements are held to be the only possible options, when in reality there exists one or more other options which have not been considered.

The dilemma need not be limited to two choices; it may involve three possibilities, in which case it is known as a trifurcation. There may be even more choices involved, in which case the fallacy may arise simply by accidental omission—possibly through a form of wishful thinking—rather than by deliberate deception.

When two alternatives are presented, they are often, though not always, two extreme points on some spectrum of possibilities. This can lend credence to the larger argument by giving the impression that the options are mutually exclusive, even though they need not be. Furthermore, the options are typically presented as being collectively exhaustive, in which case the fallacy can be overcome, or at least weakened, by considering other possibilities, or perhaps by considering the whole spectrum of possibilities, as in fuzzy logic.

From About.com

The False Dilemma fallacy occurs when an argument offers a false range of choices and requires that you pick one of them. The range is false because there may be other, unstated choices which would only serve to undermine the original argument. If you concede to pick one of those choices, you accept the premise that those choices are indeed the only ones possible.