A qubit is a quantum bit that is the basic unit of information in a quantum computer. It has something – a particle or an electron, for example – that adopts two possible states, and while it is in superposition the quantum computer and specially built algorithms harness the power of both these states. It is the quantum mechanical analogue of a binary digit or bit in classical computing.
In classical computing the information is encoded in bits, where each bit can have the value zero or one. In quantum computing the information is encoded in qubits. A qubit is a two-level quantum system. A qubit can be in any of the states or (unlike a classical bit) in a linear combination of both states. The name of this phenomenon is superposition.
The coining of the term qubit is attributed to Benjamin Schumacher. In the acknowledgments of his 1995 paper, Schumacher states that the term qubit was created in jest during a conversation with William Wootters. The paper describes a way of compressing states emitted by a quantum source of information so that they require fewer physical resources to store. This procedure is now known as Schumacher compression.
Particles in superposition is the main way to store information in quantum computers.
However, storing a quantum state – i.e. particles in superposition – is very difficult. Any interaction with the universe will disrupt it and cause errors. This is why quantum computers are shielded electromagnetically and cooled down to almost absolute zero.