Qubit is short for “quantum bit,” which is the unit of measure for quantum information. It is the quantum computing version of a binary digit or a bit.
In classical computing, bits are used to communicate encoded information. They represent two possible states represented by 0 and 1. These two states could mean “true” or “false,” “empty” or “full,” or “on” or “off.” Bits can only operate in one of two states at one time. They can switch from one state to another at a slow speed and under low electronic voltage.
A qubit is similar to a bit in the sense that it encodes information on two levels. Unlike a bit, though, a qubit does so in two quantum states at the same time. That said, you can only manipulate qubits to accomplish complicated tasks or calculations using so-called quantum principles such as quantum superposition or entanglement. Because of this property, a qubit can encode a more expansive array of information than a bit.
In Dirac notation, which is a way to describe quantum states, the following equation represents a qubit:
|0〉 + |1〉