WoWDB

These are earned weekly in stacks of 3 by turning in 90 Lesser Charm of Good Fortune for the weekly quest at your faction's city in Vale of Eternal Blossoms (Alliance / Horde). You can only earn 3 of these a week up to an overall limit of 10, at which point you need to spend them to accrue more. Whenever a player kills a Pandaria raid boss they will be prompted with a small pop-up similar to the need/greed window, and it will ask them if they want to use an Elder Charm of Good Fortune for an extra chance at loot. If a player selects "need" then they will have a small chance of receiving an item from the boss loot table that is suitable to their spec & class. If a player does not win an item with their Elder Charm of Good Fortune then they will be awarded a small amount of gold instead.  Players who do not have any Elder Charms of Good Fortune will not receive this popup.

Lesser Charms of Good Fortune are earned by completing daily quests throughout Pandaria.

Both types of charms display in a player's currency tab.

The elder charms will get a cap of 20 when the next patch hits (patch 5.2). But it can ONLY be used for patch 5.0 raids and outdoor bosses. Which means that we will get a new currency for the new raid and bosses when patch 5.2 arrives.

The replacement currency for patch 5.2 will be named "Mogu Charm of Fate", which will be usable for bonus rolls in the new raid, Throne of Thunder, and for the new world bosses, Nalak the Storm Lord and Oondasta. These new charms will still be converted from  Lesser Charm of Good Fortune .

Actually, the name of the 5.2 currency is Mogu Rune of Fate. It has also now been announced that the weeklies for the Elder Charm will be removed in 5.2, and replaced with new weeklies for Mogu Runes. Elder Charms will be obtainable only from item drops in the new 5.2 dailies zone. (source)