Merlin is the fictional famous wizard and adviser to King Arthur. He was also adviser to Arthur's father King Uther Pendragon. As the legend came from many different sources over hundreds of years, there are differing accounts.
In the earliest sources, such as Nennius, Merlin is apparently the same person as Ambrosius Aurelianus, but later these two figures became two distinct individuals.
Merlin, a native of Carmarthen in Wales, was a boy born without a father, but his father was actually a Roman consul (according to Nennius at any rate). After serving as adviser to both Uther Pendragon and Arthur for many decades, Merlin, now a very old man, fell in love with a nymph named Nimue, Maeonia, Vivian, or perhaps even Morgan la Fay, depending on which account you read. She stole his magical power and imprisoned him in a crystal cave, where he still is, it is said. In other sources Merlin's real name is Lailoken, and he lived in Scotland, and went mad after a battle and ran into the woods to live as a hermit.
The story of Merlin is still popular today, and the figure comes up in a number of different movies and games.