It is a thick fine paste that comes in a variety of colors depending on what grains are used and the amount of time it is fermented for. It has a salty taste - somewhat like soy sauce, but the flavor is different. It has great nutritional value. It is also high in vitamin A, thiamine, riboflavin, vitamin B6, iron, calcium, phosphorus, sodium, and potassium. Finally, miso is a third source of vitamin B12 for vegans.
Nutritional yeast is yellowish in color and usually comes in flakes.
You can purchase it in bulk at just about any health food store. It tastes and smells a lot like normal bread yeast, but it is different. It can be used to make a really good sauce that is the closest vegan thing to cheese I have ever found. The texture can be a lot like chicken when cooked well. Seitan can be used to make a mean vegan mushroom stroganoff.