At one point he was just a wizard. Powerful, but normal. He became obsessed with immortality and purity. He split his soul in to pieces because he found out that if he did that, his body could be killed, but his soul would live, as long as there were pieces left.
He started two different wars in our world, wanting to wipe out or subjugate non-magic folk and anyone who wasn't a pureblood wizard.
At the end of the first war, he tried to kill a baby, Harry Potter, who was prophesied to be the one who could kill him, but it didnt work because the baby's parents had died to protect him and their magic countered the spell Voldemort used, made it rebound to him nuh instead. But since his soul couldn't die he lost his body instead.
It took 11 years but eventually he came back and started the second war when he tried to kill Harry again. Eventually Harry found out about the soul pieces and was able to destroy them while the rest of us did everything we could to protect our world. There was a final battle at our school where Harry was finally able to kill him. But apparently he's back here again.