SCOBY is an acronym that stands for symbiotic culture of bacteria and yeast. It's the "mother" that you use for every batch of kombucha you make, and it looks like a round, flat, opaque-beige piece of agar agar (like what was in those Petri dishes in chemistry class). You can order a SCOBY, or you can make your own. But you have to have one in order to brew kombucha. Whenever you brew a new batch, a new layer grows onto the SCOBY; it's easy to peel off, so you can use it to start another batch for yourself, or you can give away 'baby' SCOBYs to friends who want to get started.
How to Make It
Bring water to a boil in a large saucepan. Add sugar, and stir until sugar completely dissolves. Turn off heat. Add tea bags, and allow mixture to come to room temperature (add ice to speed this process if you're in a hurry).
Pour kombucha into a 1-gallon-sized jar. Add room-temperature tea; do not add hot tea, as it will kill the good bacteria. Add cool water to fill jar to the top, just where the mouth starts to narrow. Cover jar with two coffee filters or a double layer of paper towels, and secure with a rubber band. Place jar in a place, at room temperature, where it won't get jostled and it's out of direct sunlight; it does not need to go in a dark place, just out of direct sun.
It will probably take 2 to 4 weeks for your SCOBY to form. You can lift the covering to see what's happening—just try not to slosh the liquid at all. At first, nothing will happen; then, after a few days, you'll see some bubbles forming on the surface. Then you'll see more and more bubbles, and maybe a thin, clear jellyfish-looking blob on the surface. Once the blob covers the surface, is opaque, and is roughly ¼-inch thick, you have a viable SCOBY.