The masu is a square wooden box made from hinoki (Japanese cypress). Originally, it was used to measure rice. Nowadays, the masu is used almost exclusively to drink sake. (But also used to hold beans during the bean-throwing festival of setsubun.) In Japanese pubs that serve sake in a masu, the masu will be put on a saucer and it will normally be filled to overflowing. Bonus tip: drink from the corner! (^-^)
The wood from hinoki has been used for construction in Japan since olden times. The temple Horyuji in Nara (the one that houses the big Buddha statue), founded in 607 AD and considered one of the oldest existing wooden structures in the world, is made from hinoki. (Note: The temple was built in 607 but has had reconstructions throughout history.) Hinoki is well known for its resistance to rot and Japanese people love the fragrance of the wood.