The masu (the kanji is 枡 from 木 wood and 升 literally "measure", however unspecific) is a square wooden measuring cup which holds one gou, one tenth of a sho (shou as in一升瓶; isshoubin, an ancient Japanese measure for liquid of 1.8 liters). Therefore a one sho bottle of sake (酒 alchohol with the radical for water) will fill ten masu cups just to overflowing. The cup itself is made, typically, from 25 pieces of cedar (see also: hinoki). One square bottom, and 24 slender, three-inch pieces aranged interlocking to build up the sides. The external measurements may vary (about three and a quarter per side, two inches high) but the internal volume should not.
The masu cup is a great way of drinking cold sake (one would never drink warm sake from a masu) as the wide mouth and cedar aroma enhance the sake's flavor. To drink from a masu just choose a corner and sip, holding one side with your right hand (thumb on the top and fingers on the bottom, and supporting the other side lightly with your left hand).
These days you are most likely to drink from a masu at a festive occasion, a wedding or anniversary, the New Year's celebrations, or at a kagami biraki (also known as the kagamiwari or "cask breaking ceremony")
The kagami biraki is held to commemorate the opening of a new business (what we, Americans, would associate with a Grand Opening or Ribbon Cutting). Kagami refers to the round wooden lid on a sake cask that is broken open by the guest of honor using a large, wooden mallet (kiduchi, like the ones shounen manga characters use on each other). Well what does that have to do with the masu cup? The sake is then ladled into masu cups for all in attendance and quaffed with a hearty "かんぱい!" (乾杯 kampai, for toasting). This ceremonial opening of the sake cask is believed to bring blessings of health, happiness and prosperity to all.
See also: wooden sake cup