Treasures are Secrets in Tomb Raider Underworld. There are 179 of them in the game and the amount of them in each level varies a lot. Some of them are easy to find, they might be in plain sight, others are hidden inside of Secret Vases or in places that aren't easy to reach. By collecting them the player unlocks bonus content.
In addition to Treasures there are six Relics in the game.