The principal feature of any quantum system is that is can only have discrete energy.
The fundamental (and radical) idea underpinning Q.T. (quantum theory) is that energy, momentum, spin etc. can only take defined (discrete) values, not any continuously varying value. All these variables have well defined values they can take - much like the rungs of a ladder - and all values between the rungs are forbidden.
This is weird, it's almost as if there is no physical description for an electron between two energy states, a physicist would describe this as meaning they don't exist. Mind-twisting, but true.