He was a Scotsman who supposedly came from Aberdeen. (PROSE: Prelude Human Nature) He taught history, specifically that of the Bronze Age, at Hulton College in Farringham, Norfolk. Although he lacked the Doctor's knack for plans, and was more emotionally available than the Doctor, he nevertheless displayed the Doctor's key beliefs on non-violence, being unwilling to take up guns even after seeing one of his students shot. He was briefly the lover of Joan Redfern, but this relationship was cut short when Farringham was attacked by the Aubertides, a race of creatures seeking the Doctor's biodata — contained in a pod for when the Doctor was ready to become Gallifreyan once more — to increase their reproductive cycle and allow them to conquer Gallifrey.
Although scared by the alienness of what the Doctor represented, Smith was persuaded to assume the Doctor's identity once again when a vision of "Verity" — an old relationship of his, but really an aspect of the TARDIS left in Smith's mind to protect those aspects of the Doctor that couldn't be extracted — reminded him of how the two were similar. Having returned to normal, the Doctor tricked one of the aliens into taking the pod that now contained Smith's biodata. The alien turned into Smith, who chose to sacrifice himself for Joan. After a brief conversation with the Doctor — during which the Doctor told Smith that he did rather well in his time as the Doctor — Smith was taken by Death. (PROSE: Human Nature) This fulfilled the deal the Doctor made with Death on the planet Heaven, where the Doctor offered Death one of his lives in exchange for Ace's. (PROSE: Love and War, AUDIO: Love and War)
This one belongs to me, now. I told you long time ago, Doctor, that I would take a life from you in return for that of your companion. Our business is now concluded.
Four lifetimes later, the Tenth Doctor again assumed the human alias of John Smith, this time attempting to escape the Family of Blood, an incorporeal race who sought the Doctor's DNA to increase their lifespan, via the use of the Chameleon Arch. Unlike the Seventh Doctor's Smith, the Tenth's Smith was far more a product of his time, as evidenced by his slightly racist treatment of his companion Martha Jones, while remaining fairly similar to the Tenth Doctor in general details about his personality, such as a habit of talking too much. (TV: Human Nature / The Family of Blood)