Grand Master
Grand Masters are the big cheese of the Grey Knights with only the Supreme Grand Master being more powerful. They are armed and armored in the best the Imperium has to offer and benefit from hundreds of years of experience in fighting the daemonic. They are devastating in close combat, especially when backed up by a retinue of the feared Grey Knights Terminators. While all Grey Knights are psykers, the Grand Master's skills are honed to such a fine point that he can wield his psychically attuned Force Weapon with exceptionally terrific power. The Grand Master is certainly one of the greatest warriors in the entirety of the Imperium.
The Grey Knights only have a total of eight Grand Masters at any one time. This number is derived from the eight founding Space Marines which Malcador the Sigillite brought to Titan.
Each Grand Master has final command of a Brotherhood, although operational command is often left to the Brother-Captain. The Grand Master will only take full command of his Brotherhood if there is a battle which is considered too perilous for even the hardened skills of a Brother-Captain to handle.
Grand Masters hold responsibility for maintaining the Chapter's many alliances, whether with other Adepta within the Imperium, or the various alien races with whom the Grey Knights have covert dealings. The actual detail of such treaties, pacts and accords are worked out by the Chapter's scribes -- a warrior's time is too precious to be expended on such bureaucratic detail. The Grand Master's chief role in these matters is to attend all meetings with outsiders in his full regalia of war.
A master daemonslayer, the Grand Master always wears Terminator Armor and wields a Nemesis Force Weapon in combat, and he is also a potent psyker. Few, if any, daemons have been able to stand against a Grand Master of the Grey Knights and survive. The death of such a powerful defender of the Emperor's will is always mourned by the Grey Knights and the Ordo Malleus, though no common citizen of the Imperium will ever know of his existence. Due to the amount of experience needed to become such a warrior, Grand Masters are typically ancient Space Marines who have seen nearly a millennium of service to the Emperor.
Tabletop
The Grand Master is a tank that costs a more than a regular marine commander with similar stats. Note no orbital bombardment, or even the option to take it in order to balance out the psychic mind bullets his army will take into the field. As stated, he becomes an ML2 psyker, comes with terminator armor (which you sadly can't remove if you don't want it), and can equip any of the Grey Knights' heavy weapons. He can also take all the stuff available for Grey Knights, including a bonus MC weapon, a Teleport Homer, and Digiweapons.