There are two elements to this discussion, controllability and stability. Entry level helicopters are generally very controllable and exhibit docile flying qualities while not being very stable in the classic sense. These machines use rotor inertia, stabilizer bars or a combination of the two to achieve the pleasant flying qualities we have grown accustom to. By contrast, larger more complex machines use electronic solutions for stability as Nick mentioned to achieve true stability. As an example the S-92 is inertially stabilized in a hover. It uses rate gyros, accelerometers or an inertial reference system to neutralize any tendency to drift that would be common to an un-augmented machine.