A qubit is two-state quantum system which is the smallest unit of quantum information. Its two basic states are conventionally labeled |0⟩ and |1⟩ (pronounced: ket 0 and ket 1). A pure qubit state is a linear superposition of those two states. This is significantly different from the state of a classical bit, which can only take value 0 or 1.
A number of Qubits taken together is a qubit register.
It is also possible to have a three-state system, called a qutrit, whose states are conventionally labeled |0⟩, |1⟩ and |2⟩.