There are two types of nori seaweed. Asakusa nori is harvested from bays. This is of higher quality and is used in sushi. Iwanori is gathered on shore. Both are now cultivated in nets and dried in sheets.
Nori can be kept for a long time. However, it absorbs moisture easily and should be stored in a sealed container or refrigerator. So make sure to put it away immedietly after using.
Nori is delicious whether it is used to wrap sushi as a maki (roll) or folded around rice or crumbled as a condiment.
The flavour of nori is enhanced by holding 2 sheets of nori together while waving them gently over a strong heat. Reverse sides and repeat process. Do not let the flame touch the sheets.
There is no need to do this (aburu) for yakinori ajitsuke nori because both are already flavoured. Momi nori is crumbled seaweed used as a topping. It is made by crumbling scorched (aburu) nori in your hands. When very finely crumbled, almost into powder, ao (green) nori is a great topping for okonomiyaki, Japanese omelette. It is sold in this form too. Tsukudani nori is fresh seaweed that is cooked with shoyu (soya sauce and sugar).