Unique Items can be Ethereal as well. This means that this kind of Uniques will grant additional Damage or Defense and you can only use them as long as they have enough Durability. Note that you can insert a Zod Rune (Indestructible) in an Ethereal Unique Item in order to use them for a long time. Moreover, these Items don’t lose durability while you have them equipped on your mercenaries.
You can socket a Unique Item using the 1st quest reward in Act 5. You can insert Runes, Gems or Jewels in Unique Items in order to make them even more powerful. However, Unique Items can only have 1 socket, (with a few exceptions). Therefore, it is impossible to complete a Rune Word using a Unique Item.