Stratum is used for mining pools.
NiceHash’s specification for Ethereum stratum mining protocol v1.0.0: https://github.com/nicehash/Specifications/blob/master/EthereumStratum_NiceHash_v1.0.0.txt
Ethereum does not have official stratum protocol. It supports only GetWork, which is very resource hoggy as miners need to constantly poll pool to obtain possible new work. GetWork thus affects performance of miners and pools. Due to demand for more professional Ethereum mining, several versions of “stratum” for Ethereum emerged. These “stratums” utilize GetWork on server side (pool side) to obtain work, which would be fine, if careful considerations and precautions were taken when creating such protocols.
NiceHash May 2016 announcement: https://forum.ethereum.org/discussion/7091/stratum-mining-protocol-for-ethereum
Bitcoin’s Stratum Protocol: https://en.bitcoin.it/wiki/Stratum_mining_protocol
Bitcoin’s getwork Protocol: https://en.bitcoin.it/wiki/Getwork