The black seaweed wrappers used in makimono are called nori. Nori is a type of algae, traditionally cultivated in the harbors of Japan. Originally, algae was scraped from dock pilings, rolled out into thin, edible sheets, and dried in the sun, in a process similar to making rice paper. Today, the commercial product is farmed, processed, toasted, packaged, and sold in sheets.
The size of a nori sheet influences the size of makimono. A full-size sheet produces futomaki, and a half produces hosomaki and temaki. To produce gunkan and some other makimono, an appropriately-sized piece of nori is cut from a whole sheet.
Nori by itself is an edible snack and is available with salt or flavored with teriyaki sauce.
Source: http://en.wikipedia.org/wiki/Sushi; April 19, 2014