Certain items are classified as either Bind on Pickup or Bind on Equip. Bind on Pickup means that once an item is in your inventory it turns into Bound to Player and cannot be given to another player. Most items can be sold to an NPC vendor or salvaged, but not traded or sold at the Auction House.
Player can receive BoP items via mail, for example exlusive head models arrive via mail. Player cannot however send BoP items, as those are bound on player on receiving them initially. Such items can be stored in bank, if player doesn't wish to keep them in back bag.
Most often Bind on Pickup items are: