Karashi is Japanese yellow mustard and a popular condiment used for Natto, Oden, and many other foods. Karashi has a very hot and spicy taste as opposed to western mustard which is usually milder because of being mixed with vinegar and other ingredients. Karashi is used often for dipping to enjoy spiciness.
You can buy Karashi in powder and paste forms. Powder Karashi is mixed with water to become a paste. Pre-made Karashi paste is ready to use out of tube just like Wasabi.