Half-blood refers to those wizards and witches who have magic and Muggle ancestors in their family trees. Half-bloods are the most common wizard blood, far outnumbering pure-bloods and Muggle-borns. Rowling has stated that of the Hogwarts annual intake, 50% are half-bloods. Pure-blood supremacists view half-bloods as inferior to them, although superior to Muggles and Muggle-borns.
Voldemort is a half-blood since his father was a Muggle and his mother, a pure-blood. Severus Snape is also a half-blood (he gave himself the nickname "The Half-Blood Prince"), as his father Tobias Snape was a Muggle. Harry himself is a half-blood, since his mother was Muggle-born.