A common metaphor in America for trying to find something concealed in a great field of similiar objects. Of course, it doesn't have to be an object, it can also be a person in a crowd or an idea in a sea of ideas. Although (barring gnarl's solution) finding a needle in a haystack may seem like an extreme example, anyone who has spent time in a research library may think it is an understatement.
In fact, this metaphor often doesn't convey how complicated real life situations are: in real life, after you have found the needle, you are finished, because it is pretty easy to tell what a needle looks like. In real life it is often a case of finding a piece of hay in a haystack, and then not knowing whether or not the piece of hay was the right piece of hay.