A Don is the head of an organised crime syndicate, who is the leader of a crime family. In the Mafia, they are known as a "Don", in the Triad they are known as the "Mountain Master", while in the Yakuza, they are known as a "Kumicho".
In organised crime, every single crime syndicate has a figurehead, a captain, to rule them. The boss of a family is typically a wealthy and powerful man, like a king. A boss chooses a successor, whether it is in his blood family or in his ranks. A boss gives directions to his underboss, who in turn sees that the Capos oversee that the soldiers or spys accomplish the task. The boss and his blood family usually live in a mansion, which is generally a fortress-like castle guarded by hired guns, and family, sometimes even hired cops on payroll. Bosses are advised by a Lawyer in their actions.
Bosses try not to be up close in fighting, sending their made men to carry out the bloodshed instead. In 2013, Gammossi Family boss Antonio Gammossi carried out the Gammossi-Travonni War personally, using a Dillinger and Street Sweeper to gun down several of the Travonni Family members, including Charley O'Hara and Joe Gaffney.
Some powerful mafia bosses get seats on The Commission, the legislature of the Mafia. Bosses represent their families in meetings to discuss important matters, such as the Five Families Mob War, the retirement of Sal Tessio, and the retirement of Pete Clemenza. They give permission to put hits out on people, including the bosses or made men; otherwise it is infamia.