Short answers, pulled from the story.
Big Boss was the mastermind behind the operation to destroy a bipedal nuclear tank and the commanding officer of the protagonist Solid Snake. He was revealed as the enemy to the player, establishing a legacy of betrayal that defined the franchise.
Naked Snake was sent to rescue a scientist and kill a traitor known as The Boss, but the operation was a trap set by the United States government. He killed his mentor The Boss to save his country, only to learn the mission was a lie orchestrated by his own government.
The government project used Big Boss's DNA to create three clones: Solid Snake, Liquid Snake, and Solidus Snake. This betrayal shattered his trust in the world of espionage and led him to become the leader of Outer Heaven.
Venom Snake was a medic who lost his left arm and suffered severe brain injuries in a helicopter explosion in 1975. He was subjected to brainwashing and facial reconstruction to become a body double for the true Big Boss.
Big Boss died in 1984 from exposure to a new strain of the FOXDIE virus. He was the leader of a global terrorist organization when he faced the end of his life.