In most atoms, electrons occupy a zone exterior to the nucleus. Though electrons are often represented as particles in discreet orbits around the nucleus, they cannot be said to occupy a specific point in space. Rather, their position can be described as a wave function.
The exterior shell of the atom is not the only place electrons can be found in an atom. Though the electron shell is the only stable location for electrons, a heavy atom that undergoes beta decay can emit an electron from its nucleus. To avoid confusion, electrons emitted by the nucleus are referred to as beta particles.