Put simply, ROOT Sports is the regional cable sports network carrying the Pirates, Penguins, and various other sports in and around the Pittsburgh market.
Regarding ROOT Pittsburgh specifically, they're known to carry WVU Football at such times....like last weekend, and this coming weekend......when the national networks (like ESPN and Fox) don't have the telecasts. Said another way, ROOT is the Fox Sports ________ equivalent in the Pittsburgh, Denver, Houston, and Seattle markets. However, their programming is sometimes carried (as was the case last weekend) outside their respective markets on FOX Sports regional channels.
To the best of my knowledge, ROOT does not have a native streaming service (though perhaps someone can confirm/deny).