The Deku Shield is made out of tough, high quality wood, and thus, will not break. Being made of wood, though, it will burn if it comes into contact with fire. It is very easy to lose in places with lots of fire, like Dodongo's Cavern. If Link does lose it, he can purchase another one at the Kokiri Shop, or find one in one of the many Treasure Chests scattered around Hyrule. Some Business Scrubs sell Deku Shields as well.
The emblem on the front of the shield is the symbol of the Kokiri, and is the same shape as the gold on the Kokiri's Emerald.