Nether Wart is a vanilla Minecraft plant, as well as a Tier 5 plant from Industrial Craft 2 (Tiers range from 1-8). It can be acquired from Nether Fortresses and Cross Breeding. You can plant Nether Wart into Crops directly, though their stats will all be 1.
In its initial stage of growth it is identical to Terra Wart, another Tier 5 plant.
Harvesting Nether Wart may yield either Nether Warts, Nether Wart Seed Bags, or both.
Also, if you cross breed a Dandelion with a Rose, it has a small chance to create a Nether wart, though this is rare.
Nether Wart is mainly used for brewing potions in vanilla Minecraft. Click this linkfor the Minecraft Wiki article on potions.