Groups describe symmetries of single objects. The symmetries of families of objects are described by groupoids. If we want to study all families of a certain type of geometric object (Euclidean triangles, say, or Riemann surfaces), we are led to the notion of stack. In this talk I will explain what a stack is and why stacks are useful. I will give a few simple examples. In recent years, algebraic stacks have become an indispensable tool in algebraic geometry, but other areas of geometry should profit from considering stacks, too.