Nori is a thin sheet of edible dried seaweed. The color is blackish green or blackish purple. It comes in a large sheet (little smaller than a letter size), half size or even smaller size or flakes.
Sushi can be rolled inside of a nori sheet, and flake type can be sprinkled over chirashi-zushi or other food as garnish.
It is important to keep it in a tightly sealed container to avoid moisture, otherwise it gets soggy quickly. Many value pack (30, 50 or 100 sheets in a pack) comes in a resealable bag.
When you fold over, it should break easily with a dry crinkling sound. If nori gets a little soggy, pass it lightly over a flame to make it crisp again. Overly soggy nori which can’t be cut easily by hand is no longer fresh, but it can be stewed with soy sauce and mirin to eat over rice.
Nori has a front and back. The shiny side is front, and the coarse side is back. To wrap onigiri, or roll sushi, always keep the front side (shiny side) facing out, as it makes for a better presentation.
There are seasoned nori as well. They are always cut into thin strip size, and eaten with steamed rice. Japanese seasoned nori has a sweet soy sauce flavor, and Korean seasoned nori has a salty sesame oil flavor. These are not for sushi.
Japanese nori can be bought at Japanese, Chinese, and Korean grocery stores. For those who wish to order online, here are the links;