Nori is a dried seaweed. You can buy nori as a sheets that are folded and packed in plastic with moisture eater. Usually they are very thin.
Most common usage for nori is to make norimaki-sushi, the special style of sushi, popular especially abroad. Nori is also used for making onigiri, the traditional Japanese lunch. Pieces of nori are also found in ochasukenori soup.
Nori production is big business in Japan. Comparing to end of WWII, the nori harvest is now six times that large. There are said to be about 60 000 fishermen who cultivate nori. Some of them do this aside their normal fishing.
There are various techniques for culturing nori. Early techniques included setting bundles of twigs in estuaries. The spores settled and grew in those twigs. Nowadays, nets are used to collect the spores. The nori is cultured both in tanks and in sea.
There are problems in Japan's nori production. One is the inevitable labour shortage, especially when young people and sons of fishermen families escape to other industries, such as the IT field.
Nori is a very healthy food and is famous among dietists.