The best father and one from the best hunters

John Winchester was the father of Sam Winchester and Dean Winchester, Adam Milligan and the husband of Mary Winchester.

Background
John began his hunting "career" on approximately November 2, 1983, following the death of his wife. His primary goal, as a hunter, was to kill the Yellow-Eyed Demon: Azazel, for murdering the mother of his children.
John is a native of Lawrence, Kansas. He left to fight in Vietnam as a Marine, he held the rank of Corporal. He later returned to Lawrence where he worked as a Mechanic. On April 30th 1973, John bought a 1967 Cheverlot Impala second hand at Rainbow Motors, after being convinced by Dean (Whom unbeknownst to him, was his son.)
John had a history with Mary Campbell, although before falling in love (under the influence of a Cherub), they couldn't stand each other. John intended to propose to Mary, however he was killed by Azazel (Who was possessing Mary's father Samuel.) Mary then made a deal to bring him back. In exchange for him to be allowed to give Sam his blood. (This condition was unknown to Mary.)
After Mary died, John went to a psychic named Missouri. He learned the truth about Mary's past and of the Supernatural. He left Lawrence with his sons Sam and Dean. He dedicated his life to hunting down the Yellow eyed Demon.
John accumilated a vast knowledge of the Supernatural, writing a journal of the creatures he hunted. He was a well regarded hunter by his peers. John worked with several other hunters. Most notably, Bobby Singer, William Harvelle and a priest named Caleb.
His life as a hunter led to strain between him and his son Sam, who wanted to quit hunting, leading to an arguement where John told Sam if he left he would have to stay away. However John regretted saying it, as he didn't want Sam to be on his own. John would even stop by Stanford every so often just to make sure he was all right, (Unbeknownst to Sam.) John Admitted it was never his intention to raise his sons the way he did.
Around August or September of 2002, after Sam had left for Stanford University, John recieved a call from Kate Milligan, a nurse who had treated John several years previously in Windom, Minnesota. She informed him that he had a son by her named Adam who was twelve at the time. According to Adam, John "dropped everything" and drove all night to Windom. After the initial meeting, John had sporadic contact with Adam, he taught him to play poker, taught him to drive in the Impala and bought him his first beer when he was fifteen. He also took him to ball games on his birthday. It can be suggested that perhaps John was trying to replace Sam with Adam who was away at Stanford when he first met Adam. John never mentioned Adam to either Sam or Dean who only learned about him around two years after John's death.

Family
Mary Winchester (Wife)
Dean Winchester (Eldest son)
Sam Winchester (Second son)
Adam Milligan (Youngest son)

Jeffrey Dean Morgan as John Winchester
SPN - Somewhere [John Winchester]
Supernatural- Dean/Sam/John relationship