???

If this is supposed to be   {  }  ......it indicates the "empty set"........it usually means that no solution to a problem exists........

I really depends on where you saw it.

If you saw it stand alone then Yes it is probably an empty set just like CPhill said.    

But parentheses are used for other things too.

Like in LaTex, the arguments of a function are indicated by putting them in parentheses :)