Harry Potter and the Order of the Phoenix?

I don't understand why the hard *** teacher from the ministry of magic goes there, why do the rest of the students train with harry i mean Voldemort and his followers are only going after harry really right and maybe some mud bloods, oh and could you please name the followers of voldemort? One more thing, how did Harry and them know voldemort was going after them, knobody really spotted them or saw them cauing havoc , like they just pretty much all of a sudden started training