"Pureblood" is a term used to describe a vampire with a pure lineage of vampire ancestors. It isn't always 100% pure, but it's close enough to warrant the title. Pures tend to be more or less demonic or powerful depending on what generation they belong to. The first generation are more akin to the harpies and demons of old vampire lore, and the later generations tend to be more human in appearance and weaker in comparison.
Pure blood vampires are not vampires that were once human and got "turned" but rather they are vampires that were born as vampires. A vampire infant will grow like a normal human would up until he reaches his late 20's to 30's, where he will stop aging altogether. A pure vampire who ages beyond that has simply not taken very good care of himself, as drinking blood should rejuvenate a vampire and keep him in his prime.
Pure vampires and "spawned" vampires can have children, letting a little bit of human blood get into the bloodline, but spawned vampires together are often unable to reproduce. Vampires who get it on with humans will end up with Dhampirs if the mother is human, and 4th generation vampires if she's a vampire.
The Vampire King Rodericke is a Pureblood with some obvious human bloodlines.
Pure Bloods with human lineage are easy to tell apart from "spawn" vampires because they have pointy elf ears, and sometimes strange eyes.
The vampires that are more "pure" than the self-proclaimed "pure blood" are called Ancient vampires, this refers of course to their lineage and not their actual age.