A physical (quantum-mechanical) system that can be configured in (at least) 2 different states (i.e. 0 and 1). It is used in quantum computers to store the information of 1 bit. Because it obeys the rules of quantum mechanics, it can be prepared in a superposition of the states 0 and 1 at the same time
Electrons serve well as qubits because they are spin 1/2 particles. Spin 1/2 particles have two (!) distinct spin eigenstates which can be used to encode binary information.