Wrath of the Lich King introduced not only new wow gold content, but an entirely new type of item. These items are called Heirloom items, or Bind to Account (BoA) items. They aren't tied to any one character and can be freely passed from alt to alt, but they're all tied to one account, the account that bought the item. No handing them over to your friends, no mailing them to other players, no mailing them to your second account. So far none of these items are drops, but rewards for more veteran players of the Wrath content. There are two different ways of earning these items, but they dovetail nicely.

Quite simply, you need to participate in Wrath's content. Items suited to PvE are acquired via Emblems of Heroism, and items suited for PvP are acquired via Stone Keeper's Shards.

Emblems of Heroism
Emblems of Heroism are one of the Badge of Justice equivalents in Wrath of the Lich King. They're acquired through beating Heroic bosses and 10-man raids. 25-man raids award Emblems of Valor, which are not used for any Heirloom items currently.

Unless you're a chronic altaholic, you may want to hold off on buying these Heirloom wow gold items, because there's also a wide variety of epic gear you can buy with it that's Bind on Pickup, not only the Heirloom items. Again, these are the Badge of Justice equivalents. Think of the Heirloom items like the epic gems or Nether Vortices from The Burning Crusade. If you want to gear up your main, these items are going to be things you buy after you have nothing else left to buy. If you spend most of your time leveling up alts, these items may be very good for you to buy right away.