John and Mary
Mary and John Winchester's relationship was complicated due to Mary's hunting background. They gradually fell in love and married, later becoming the parents of Sam and Dean Winchester. When Mary was brutally murdered by Azazel, it forced her husband to go on a quest to find out what killed his wife, and when he learned that it was supernatural, he dove head first into the hunting world - exactly the kind of life that Mary tried so hard to protect him from.
Background
There were orders from Heaven to get John and Mary together in order for Sam and Dean to be born. At first John and Mary could not stand each other, but soon a cupid came in and worked his magic. Even with the cupid's powers, the love felt between John and Mary was very real. Mary didn't tell John about her hunting background. To her, John was normal and could give her that normal life she craved. She ignored her father's misgivings about John, telling Dean, her future son and who was traveling to 1973 from the future, that she would run away if she had to, but that she was in love with John and that even though he had been through the Vietnam war, he still believed in happily ever after and was kind.Pre-Dean and Sam
In 1973 (In The Beginning), John and Mary are still dating. John tells Mary he knows her father doesn't like him very much. Mary tells John that her father just doesn't know John yet. Around this time John is also thinking of proposing to Mary (Mary knows this which she reveals to Dean). Mary is also very protective of John and attacks Dean when she notices him following them.Later on, Mary calls John and asks him to take her away. They drive off towards the waterfront. There, John proposes to Mary, then "Samuel" arrives and pulls John out of the car. John tries to reason with "Samuel", but gets his neck snapped. As Mary holds John lifeless body, yellow eyes makes Mary a deal. He will bring John back if she gives him permission to enter her home in 10 years. Unable to live without John, Mary agrees. John re-awakens in Mary's arms and is confused about what happened.
In 1978 (The Song Remains The Same), John and Mary are now married and living normal lives. John works as a mechanic, and Mary stays at home. Then Dean and Sam turn up claiming to be members of Mary's family. They tell Mary about an angel coming to kill her and John. While they are talking, John gets lured away by Anna and Mary comes to John's rescue. John finds out about Mary's hunting past and is angry she never told him.
Mary takes them all to a safe house and they prepare for the angel. During this time, Mary reveals to the boys that shes pregnant with Dean. Anna comes in and knocks John away. Mary comes to John's defense, but is overpowered by Anna. Just as Anna is about to deliver the kill, Michael (possessing John) kills Anna. Michael renders Mary unconscious as well as wiping hers (and John's) memory of ever meeting Sam and Dean. Michael reveals that John said yes to Micheal when he told John he could save Mary. Later on, a pregnant Mary and John stand in the nursery looking an angel statue Mary bought. She tells John everything is going to be okay.
"Ooh – quite a kick there. Troublemaker already? It’s okay, baby... It’s all okay. Angels are watching over you." — Mary Winchester to Dean
Dean is born on January, 24 1979. Between that time and Sam's birth, John and Mary's marriage strains but they make amends. Four years after Dean's birth, Sam was born on May 2, 1983.
Season 1
Mary and John are now parents of two. It has been six months since Sam has been born. Mary and John are putting the boys to bed. In the night, Mary notices some noise in Sam's nursery and goes to check on Sam. There she see a figure over Sam's bed, and thinks it's John. She heads towards bed and then she notices a light flickering. Mary also notices that the television is on downstairs and that John is sitting in front of it. Mary runs back to the nursery.John wakes to Mary's screams and runs to Sam's nursery where he find her on the ceiling. John grabs Sam and gives him to Dean. John returns for Mary, but it's too late. The room is engulfed in flames. John is devastated and vengeful after Mary's death and tries to figure out what killed her. He meets Missouri Moseley who shows John the supernatural side of the world and what really killed Mary. From there out, John becomes a hunter and starts hunting the Yellow-eyed demon responsible for her death.
Even after Mary's death, John wears his wedding ring. In Home, John is surprised to learn that Mary's spirit protected Sam and Dean.
Source: Supernatural Wikia