Merlin is a TV series that premiered in 2008. Produced by Shine Limited, the show was aired on BBC One in the UK and NBC (Season 1)/SyFy Channel(Season 2) in America.
In the show, Uther Pendragon is still king and magic is outlawed. The druids are being hunted down. Merlin is sent to Camelot to secretly learn magic from Gaius, the court physician. While there, he finds a dragon imprisoned under the castle. The dragon says that Merlin must protect Arthur. After saving Arthur's life, Merlin is appointed as his servant. They don't like each other at first, but grow close. Merlin also has to keep his magic secret from Arthur.
Merlin isn't the only one with magical abilities. The king's ward, Morgana has the gift of prophetic visions that she keeps secret. Guinevere, her handmaiden, also knows of her gift and keeps it secret.
Most of the show revolves around Merlin, Arthur, Morgana, and Guinevere. The show has taken some liberties with the characters. Merlin is the same age as Arthur who is a bully rather than a paragon of chivalry while Guinevere (to begin with) is a common servant.
Extensive information on this version of the Aurthurian legend can be found on the Merlin Wiki.