Practically from the time you step off the plane you will see masks of different sizes, shapes and colors. Whilst these days many masks are produced solely as tourist souvenirs not to take masks seriously in Bali would be a mistake. When a Westerner dons a mask at a party it is to pretend he is someone else. For Indonesians - especially Balinese to don a mask is to begin living the life of the mask. Masks used in performances of sacred dances are sacred and as such revered and kept so in temples. You will not find such masks sitting idly in a store waiting to be purchased. That being said, there are still fine masks - all works of art - that are no longer used available for sale (for a price).
Without going too deeply into (there are entire books devoted to the subject) most masks you will see are representations of Ramayana Epic. The masks with refined features such as a short pointed nose and white face represent Noblemen, while those with garish features - red faces with bulbous noses and missing teeth represent buffoons, idiots, liars and cheats. If you look closely at the eyes you will be able to see Chinese characters as well - they are the ones with the thinner eyes.
Lastly, today masks are being made in modern shapes like a crescent moons and painted abstract colors but these are really only for souvenirs and decoration. A fine mask properly displayed and well lit adds a lot to any home or office. Most of the best masks can be found in the village of Mas.