SCOBY stands for "Symbiotic Culture of Bacteria and Yeast."

SCOBYs are primarily made up of cellulose, which is a complex carbohydrate produced by the bacteria in the culture. The cellulose forms a gelatinous substance (a mat in terms of kombucha or vinegar mothers, jelly crystals for water kefir, and jelly cauliflower like grains in terms of milk kefir) that gives the SCOBY its characteristic appearance and texture. The bacteria and yeast in the culture live within this cellulose matrix, creating a symbiotic relationship that allows them to metabolize sugars. 

Get yourself some SCOBY pets and get culturing!