The Basis of Lightning Technology is a standardized technical specification for the implementation of the Lightning Network. The standards define how various implementations can interoperate to interact with and create the same network. The specifications are currently a work-in-progress, and are continually being iterated upon as projects like lnd and c-lightning are built in parallel.
Details
The Basis of Lightning Technology (BOLT) documents describe a layer-2 protocol for off-chain bitcoin transfer by mutual cooperation, relying on on-chain transactions for enforcement if necessary.