Nether Wart is the base ingredient of all potions except the Potion of Weakness, which is brewed with a Fermented Spider Eye and a Water Bottle. When brewed into a Water Bottle, Nether Wart makes an Awkward Potion, the potion from which all others may be brewed.
Nether Wart only grows on Soul Sand, as it can be found in the Nether Fortresses. While growing, it goes through 4 stages, though the middle two result in no visible change. When Nether Wart is ready to be harvested, it is large with bulbous ends, like the left-most plant in the photo above. Jumping on the Nether Wart does not uproot it, unlike other Overworld plants. Light levels and water do not affect its growth. The Soul Sand does not need to be tilled with a Hoe to plant Nether Wart. Breaking a fully matured Nether Wart yields multiple in return, which can then be planted or used in potion brewing.