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. Other examples are available on BTC Guild's website. See also on the forums and the official site.
Tables showing miner/server support for Stratum mining protocol:
|Server||get transactions||dynamic difficulty||job frequency||resume|
|HHTT||No||No (User selected difficulty)||30s||No|
|50BTC||No||No (User selected difficulty)||5s||No|
|Ozcoin||No||Yes or user selected||30s||No|