An Ancient vampire is any vampire that belongs to the purest and oldest lineage of vampires. Unlike so-called "Pureblood", there is no human bloodline in their ancestory. Ancients have more demonic-humanoid forms, and sometimes traits such as glowing eyes and facial markings. Because of their appearance they are sometimes mistaken for Elites, which they have really little in common with. But as far as vampires go, Ancients are the most powerful beneath Elites. They do not share all of the same weaknesses as other vampires and have advanced versions of normal vampire powers (like hypnotism and vermin commanding).
Ancients are intensely magical and dangerous, but there are very few of them left as the bloodline mixes with other vampires and their kind get killed by groups of vampires.
Ancients are hunted down by vampires for their blood, which can give a normal vampire an "upgrade", so to speak. Not to mention that this is like vampire drugs for the normal vamp.
Quixoto is one of the more well known Ancients in the story. He has intense glowing eyes and wicked psychic powers.