Hand of the King
The Hand of the King is the king's chief advisor and executor of his command in the Seven Kingdoms.
Duties of the Hand include commanding the king's armies, drafting laws, dispensing justice, and generally managing the day-to-day running of the kingdom. The Hand may act on the king's behalf when he is indisposed, and sit the Iron Throne while doing so. The Hand also sits on the small council and will run the council if the king is not present.
...My shield, my stalwart, my strong right hand 
The office typically makes the Hand the second most powerful man in the kingdom next to the king, though some Hands had the reputation of ruling their kings.
It is considered by some to be a difficult and unglamorous job of great responsibility. A popular saying goes, "What the king dreams, the Hand builds," but the lowborn say, "The king eats, and the Hand takes the shit." Many able men have failed in the office throughout history.
The Hand resides in a tower of the Red Keep called the Tower of the Hand. His badge of office always includes a hand, though each Hand can design his own. A necklace of hands linked into a chain is one possible badge. Servants of the Hand wear the emblem of a hand.
List of Known Hands
A list of some of the worst Hands, Whose names are remembered as a warning to future generations.
Hands of Claimants to the Iron Throne
- Lord Corlys Velaryon, for Rhaenyra Targaryen
- Lord Mace Tyrell, for Renly Baratheon before Renly's death.
- Lord Alester Florent, for Stannis Baratheon, later removed and executed for treason.
- Lord Davos Seaworth, for Stannis Baratheon
- Ser Barristan Selmy for Daenerys Targaryen
- Lord Jon Connington for Aegon VI
The Hand of the King was the second-most powerful man in the Seven Kingdoms. He spoke with the king’s voice, commanded the king’s armies, drafted the king’s laws. Robert was offering him a responsibility as large as the realm itself. It was the last thing in the world he wanted.
"A weak ruler needs a strong Hand, as Aerys needed Father. A strong ruler requires only a diligent servant to carry out his orders."