According to John Smith's memories, he was born circa 1880 to Sydney Smith, a watchmaker, and Verity Smith, a nurse. He was raised in the Radford Parade district of Nottingham, on Broadmarsh Street. The Smith family retained a maid, Martha Jones, who continued to serve John after the deaths of his parents.
John received an excellent classical education in a school similar to the one he taught at in 1913, and became a schoolteacher himself. A talented artist, he studied art in "Gallifrey", which is presumably in Ireland. John took a new job in the autumn of 1913: history teacher at the Farringham School for Boys, in Farringham, England.
All of this is a lie.
Not long after arriving, he began having strange and vivid dreams: that his maid Martha was in fact a medical student from 2007, and that he himself was an adventurer in time and space known only as the Doctor. Fascinated by the intensity and strangeness of his dreams, John began to keep a Journal of Impossible Things, in which he recorded the adventures as the alien Doctor, insofar as he could remember them. He illustrated with rough sketches.
By November of 1913, John had begun telling others about his dreams: first his maid Martha; then the school's nurse, Matron Joan Redfern with whom he was surprised to find himself falling in love. Though in his mid-30s, John had never before been in a romantic relationship.
His courtship of Joan, however, was interrupted by an increasingly bizarre series of events centered around himself, and it became increasingly evident to John that, deny it though he would, he was not entirely human. His dreaming-self, the Doctor, was the true self; he was, as he put it, "just a story". Nothing which happened before his arrival at the school with Martha ever truly happened--except perhaps his studying art in Gallifrey (not in Ireland).
Eventually to stop the Family of Blood, Smith decided that he must become the Doctor again, even if it meant that he would "die". At one point Smith envisaged a possible future in which he and Joan marry, have children and grandchildren, grow old, Smith ultimately passing away from old age, secure in the knowledge that everyone whom he loved was safe.
When the watch opens, however, instead of finding himself...nothing, he awakens on the train to Ruby City.
Smith's humanity makes him far more emotional than the Doctor, whose actions often seemed cold or ruthless He was aghast at Martha's revelation that the Doctor was indifferent towards or perhaps incapable of love; terrified of Tim Latimer's description of the Doctor's alienness; and, finally, shattered that becoming the Doctor meant that his own life as a separate entity would end. After John 'passed away', Joan Redfern held the Doctor in some degree of contempt for choosing to come to England in 1913, lamenting that 'if he had never chosen this place on a whim...would anyone here have died?' John was, she told the Doctor, braver than him for choosing to die to protect the people he loved, rather than hide away. Unlike the Doctor, John was willing to take up weapons when the situation called for it, although he never did fire the weapon he took up.
Gifted with a new humanity and a lease on life presumably longer than the one the Doctor intended, he guards his life and humanity fiercely, and shares similar opinions of the Doctor that Joan does--although perhaps tinged more with jealousy and bitterness that he had to choose to give up his own life for the sake of a man that apparently showed no concern for it.
His sensibilities are much as any English gentleman of 1913 would be, but tainted somewhat by the Doctor's own beliefs, and heavily influenced at times by residual affection for those that the Doctor was attached to at his creation. While not possessing the mind of a Time Lord, he is still quite brilliant, capable of incredible calculation, and possesses a shockingly good aim.
Theoretically, if the concept did not completely terrify him, he might possess a knack for piloting the TARDIS, though not much of the basic knowledge required to pilot it with any competency.