It's basically just a colloquial term for all Russian-language websites. Just like the English-speaking Internet has various social news and blog sites that almost "glue" it together (Digg, Reddit, Y.C, Facebook, 4chan, etc), Runet has the appropriate equivalents (Livejournal, VKontakte, dirty.ru, 2ch.ru, etc)
I believe the Russian ISPs are highly interconnected, so Russian users get more bandwidth and less latency when they use local sites vs international.