Nutritional Yeast is a deactivated yeast so it will not grow or expand like bakers or brewers yeast. It is grown mostly on molasses or sugar cane. It is harvested, washed and then heated (to deactivate it).
Nutritional Yeast (Nooch - as it is lovingly called by frequent users) has 8 grams of complete protein in 1/4 cup with only 60 calories. It is also fortified with tons of B vitamins including B12, which is usually difficult to get on a plant based diet. B vitamins are essential for energy and a healthy metabolism. It is gluten, dairy and sugar free.
-sprinkle it on pasta or salads like you would with parmesan cheese
-use it to add a "cheesy" flavour to dressings, dips and sauces
-I like to sprinkle it on popcorn and on top of baked potatoes
You can usually find it in the "health food" section of the grocery store or definitely find it at a health food or natural food store. It may be in the bulk section. My favourite brand is Bob's Red Mill.