Quantum entanglement is a uniquely non-classical correlation between qubits that Einstein famously called "spooky action at a distance." When two qubits are entangled, measuring one qubit instantly determines the outcome of measuring the other, no matter how far apart they are. This correlation is stronger than any possible classical correlation, a fact rigorously proven by Bell's theorem and experimentally verified by Alain Aspect, John Clauser, and Anton Zeilinger (2022 Nobel Prize in Physics).
In quantum computing, entanglement is created by two-qubit gates such as CNOT or CZ. It is not merely a curiosity — entanglement is the computational resource that gives quantum computers their power. Without entanglement, a quantum computer can be efficiently simulated by a classical one (the Gottesman-Knill theorem shows that Clifford circuits, even with superposition, are classically simulable). It is the combination of superposition and entanglement that enables the exponential state space exploration underlying quantum speedups.
Entanglement is also the basis for quantum teleportation, quantum key distribution, and quantum error correction. In surface codes and other QEC schemes, entanglement between physical qubits creates the redundancy needed to detect and correct errors without destroying the encoded quantum information. The ability to generate and maintain high-quality entanglement across many qubits is one of the primary benchmarks distinguishing near-term NISQ devices from future fault-tolerant quantum computers.