BB84
The first quantum key distribution protocol, and still the most used — published in 1984 by Charles Bennett and Gilles Brassard.
The sender transmits individual photons, each polarised according to a randomly chosen basis. The receiver measures in a basis of their own, also chosen at random. Afterwards they compare openly which bases they used and keep only the bits where the bases happened to match — about half.
Security comes from the fact that an eavesdropper must measure to learn anything, and measuring in the wrong basis destroys the state. The two parties can therefore estimate the error rate in their shared material: if it is too high, someone was listening, and the key is discarded. The protocol does not prevent eavesdropping — it detects it.