Stratum is a proposal for an open source client-server "Overlay" protocol that enables thin clients (currently used by Electrum client). Stratum protocol is also used for pooled mining as a replacement for obsolete getwork protocol. Full mining service specification is available on Slush's pool website. See also on the forums and the official site.
Tables showing miner/server support for Stratum mining protocol:
|Server||dynamic difficulty||Job frequency|