Short answers, pulled from the story.
Big Boss is a fictional character and primary antagonist who commands a mercenary nation known as Outer Heaven. He serves as the enemy leader who gives radio instructions to Solid Snake before revealing himself during their final confrontation.
Naked Snake completes his mission in 1964 but rejects the Big Boss title awarded for these actions and eventually retires from active service. The original Big Boss awakens from a coma in Cyprus during 1984 to work on developing true Outer Heaven.
Venom Snake leads a mercenary unit called Diamond Dogs near Seychelles while fighting Skull Face and White Mamba. This new identity draws attention away from the true Big Boss who remains hidden until awakening in 1984.
Hideo Kojima created Big Boss with visual inspiration drawn from actor Sean Connery for Metal Gear 2: Solid Snake. Yoji Shinkawa later replaced that design when ports of the game were re-released.
Kiefer Sutherland replaced David Hayter as the voice of Big Boss for Metal Gear Solid V: The Phantom Pain announced on the 6th of June 2013. Sutherland provided both vocal performance and facial capture for the character across Ground Zeroes and The Phantom Pain.