There are many kinds of quantum light source, each producing a distinct kind of quantum light. ‘Quantum light’ first considers sources that generate the primitive quantum state of light—a single photon. Such light sources are the workhorses of the field of quantum optics, which uses the quantum mechanical features of light to explore the foundations of quantum physics, as well as to enable new kinds of information processing technologies. Squeezed light and quantum entanglement are also discussed. Entanglement is a very weird concept, but it enables a raft of information processing approaches that cannot be replicated by sending classical waves back and forth, and promises a future quantum Internet.