The World of Warcraft API uses rather complex values for some arguments and return values. Rather than re-explain them every time they're used, we document them as distinct data types (though in reality they're numbers and strings).

Complex World of Warcraft API Data Types

actionSlot - Action button slot numbers (120 in total)

auctionSortId - An identifier for sorting columns in the auction house.

auctionTypeId - An identifier specifying which type of auction to operate on.

bagId - Numbers representing bags you carry, bank bags, bank window, etc

bagType - A bittype identifier representing the type of bags an item can go into or the type of items a bag can carry (New in 2.4.0)

chatTypeId - An identifier for the different chat destination types.

emoteToken - System names of voice/action emotes

enchantString - An enchant id.

enchantLink - A string that will be clickable, if shown in-game, contains an enchantString.

inventorySlotName - Names of inventory slots.

inventorySlotId - Current mappings of slot names to numbers. May change.

itemEquipLoc - String representation of where an item may be equipped ("INVTYPE_HEAD", etc)

itemString - An item id with data about enchants, "of" type bonuses, and the item's creator.

itemLink - A string that will be clickable if shown in-game, contains an itemString.

itemType - String classification of an item, ("Armor", "Consumable", etc)

lootMethod - An identifier for the different looting methods (group loot, need before greed, etc)

playerName - The name of a player.

questLink - A string that will be clickable, if shown in-game, contains a questString.

questString - A quest id with data about the quest, including the level and quest title.

raidIndex - A number between 1 and 40

standingId - Numeric encoding of faction standing.

unitId - An identifier which specifies one of the units the API may reference ("target", "party1", etc)

unitFlag - A bittype indentifier for the relationship between the player and a unit in the combat log (New in 2.4.0)

Category:API types has the full list. (Though if something there is not available here: please list it!)