Christopher Columbus, born in 1450 or 1451 in Italy, served on a merchant ship since he was a teenager and studied mathematics, astronomy, cartography and navigation at university. At that time, the spice trade in the East was quite lucrative, but required an arduous trip eastward, around the Cape of Good Hope, to get to India, thereby increasing the price of spices in the Western World. Columbus, like many astronomers and mathematicians before him, believed the world was round. This was nothing special. What was dangerous was that he thought the world was considerably smaller than it is and thus assumed he could simply sail west from Spain directly to India, reducing the time and supplies needed for the journey. This was uncharted territory; he had no way of knowing (yet) that two formidable continents and massive oceans lay between him and his goal.
He undertook his first voyage in August of 1492; nine days later, he made landfall in the Bahamas. He sailed from one island to the next in search of any booty that would make the trip worthwhile, eventually leaving 40 of his men on Hispaniola and returning to Spain.
He returned in 1493, helped enslave natives to rebuild the settlement he'd left behind, then returned with 500 slaves for Queen Isabella as a gift. She rejected them, as she believed that any people in a land Spain had claimed were therefore Spanish citizens.
In 1498, he went back, visiting the South American mainland and Trinidad before returning to Hispaniola. His brothers, who had been left in charge, had mismanaged the settlement and there had been a revolt by the colonists. Columbus himself was returned to Spain in chains.
He was stripped of his titles but allowed to return to the Americas in 1502, where he made it to Panama before having to retreat from incensed natives, who managed to disable two of his four ships, after which he returned to Spain and died an ignoble death in 1506.
We commemorate him today for discovering a land teeming with humanity and cultures the Western world had hitherto been unaware of, of kicking off the American slave trade, and his inhumanity regarding the natives--wholesale slaughter and slavery--as well as introducing to some interesting Old World diseases that decimated their numbers by the millions.