Tom Marvolo Riddle, Jr./Lord Voldemort - Born of a witch mother and a Muggle father, Tom Riddle Sr. in 1927. His mother had fallen in love with his father, who was a Muggle. When she was pregnant with Tom Jr., she told the father that she was a Witch and he left her and moved back into the Riddle house with his parents. Tom Riddle Jr.'s mother died giving birth to him and he was raised in a Muggle orphanage. He was a powerful wizard, however, and he excelled at Hogwarts, winning awards for service to the school and magical merit. He was head boy in his seventh year. But he was already dabbling in the Dark Arts while still at school. Some of his closest friends in Slytherin were already calling him Voldemort.

After leaving Hogwarts, Riddle officially became Lord Voldemort. He gathered supporters together under the sign of the Dark Mark and called them Death Eaters. He spent years trying to discover the secret of immortality, and some of the experimentation he did on himself apparantly had some effect when he attacked and failed to kill Harry Potter. The rebounding spell didn't kill Voldemort, but it did rob him of his body and it wasn't until thirteen years after his fall that he managed to regain some of his power. 

Voldemort is one of the most powerful and feared Wizards of the age. He fears only Dumbledore, and it is Dumbledore and Hogwarts which most firmly stand against him.

Tom Riddle, Sr. - Father to Tom Riddle Jr. A rude person who dumped his Witch wife when she revealed that she was magical. He had nothing to do with his son. Tom Riddle jr. killed his father along with his paternal grandparents fifity years ago. Some of Tom Riddle Sr.'s bone was magically removed from the grave to add to the potion that allowed Voldemort to rise again.

Grandparents: Mr. and Mrs. Riddle - Paternal grandparents of Tom Riddle Jr. Killed by him fifty years ago.

the mother - fell in love with Tom Riddle Sr. He left her when she revealed that she was a witch. She died giving birth. Her last name just may have been Marvolo, although there is nothing at all to verify that in the books.