Proof Of Work is a hash that requires significant computational power to find.
In Bitcoin, miners must find a numeric solution to the SHA-256 algorithm that meets a network-wide target, the difficulty target (10 minutes).
Other PoW algorithms:
- Equihash (ZCash)
- Ethash (Ethereum)
- Scrypt (Litecoin, etc)
- Lyra (VertCoin)
- X11