Nearly everyone is overtired or overworked from time to time. Such instances of temporary fatigue usually have an identifiable cause and a likely remedy.
Unrelenting exhaustion, on the other hand, lasts longer, is more profound and isn't relieved by rest. It's a nearly constant state of weariness that develops over time and reduces your energy, motivation and concentration. Fatigue at this level impacts your emotional and psychological well-being, too.