Just what is 마걸리?

Makkoli (마걸리) is a rough equivalent to rice beer, its an alcoholic drink made from fermenting rice.  It is similar in process to beer, and in alcohol percentage as well.  The drink looks just like milk and has surprised a many of people as the taste is not very similar, however calling it rice milk wouldn't be too far off.

The drink is made by first using a mold that converts the complex carbs into easily fermentable sugars with a Koji mixture called Nuruk in Korea.  Next the yeast you add, or a wild yeast can settle in and ferment the sugars preventing other bacteria or mold from infecting the mix.  It is quite an easy process used for thousands of years.

The taste that results is generally grainy with fruit and nut hints.  It can range anywhere from sweet to downright sour depending on the strains of yeast or nuruk used and the ingredients fermented.
In Korea there are many different kinds available commercially and I recommend that you give them a try to see what you would want to make.