What are “Megaservers” ? Keep seeing this being thrown around. What are the best Horde and Alliance US East servers/realms?

I frequently have been seeing people mentioning and talking about “megaservers”. What exactly are they? Are they a cluster of multiple servers. What are the best Horde and Alliance servers for pushing and running content? With LFD tool confirmed not being implemented in WOTLK Classic, a heavy populated server would be nice for dungeons and raids. Also Wintergrasp would be fun on a balanced server I would guess. Am open to both PVE and PVP. Grobbulus just happens to be the only RPPVP server. How is Grobbulus?