The Doctor is a 900-year-old Time Lord from the planet Gallifrey.
He has two hearts, and when his body gets worn out he can "regenerate" and change his appearance.
He travels through space and time in a machine called the Tardis. He carries a sonic screwdriver, which can help him get out of sticky situations!
His most recent long-term companion was Donna Noble. She travelled in the Tardis for a while, until a clash with big baddies the Daleks.
At one point she adopted bits of the Doctor's brain and thoughts, helping him to defeat the creator of the Daleks Davros.
That came at a cost to Donna, and the Doctor had to make her better by wiping her memories of ever knowing him or travelling in the Tardis.
But the Doctor never spends too long on his own, and always seems to find a friendly face or two during his adventures.