The Grand Commendation for a faction works like this - once one of your characters reaches revered with a faction, they can purchase one of these Commendations, which once used will then allow all characters on the account to gain double reputation with said faction. Originally, this was going to simply be automatically implemented, but Blizzard decided that didn't feel obvious enough and wanted something in game to allow players to be aware of the changes. It should be noted these are as yet only for Mists reputations.
In addition to all the current Mists of Pandaria reputations, the new Horde and Alliance factions the Dominance Offensive and Operation: Shieldwall will also have Grand Commendations. In addition to making it easier for alts to level their reputations, it seems to me that these will also make the climb through revered to exalted easier as well - the text on the Commendations does not exclude the original character from the reputation gains, after all. Head over to Wowhead for the complete list of Commendations by faction.
Mists of Pandaria is here! The level cap has been raised to 90, many players have returned to Azeroth, and pet battles are taking the world by storm. Keep an eye out for all of the latest news, and check out our comprehensive guide to Mists of Pandaria for everything you'll ever need to know.