Is there such a thing as a pureblood?

Obviously there is like the Black's, Malfoy's, and Weasleys, but arent REAL purebloods so rare that the real reason Voldemort is evil because his plan of ridding the world of half-bloods and muggle borns would include practically killing everyone? Am I right in saying this? If not, then why is the whole wizarding world freaking out when only some of them aren't pureblood?