Original: Since electrons are fermions, the Pauli exclusion principle forbids them from occupying the same quantum state, so electrons have to "stack" within an atom, i.e. have different momenta while at the same place.
Simplified: Only fermions (examples are protons, neutrons and electrons) are bound by the Pauli exclusion principle, while bosons (an example is a photon - light beam) are not. A more precise way to describe the Pauli exclusion principle is to say that two of the same kind of fermions can not have the same quantum numbers.