Black hat seo - what is it?

and it's not always only your own site that pays. buy links from a network, and when google catches the network every site getting links from it loses that value at the same time. you can do nothing new and still drop overnight.

Anything that tries to game Google rather than earn the ranking - bought links, hidden text, cloaking, spun content. Works until it's caught, then the whole site can drop.