Onigiri are made of regular Japanese rice (not sushi rice) and a little salt. Common fillings are tuna mayonaise, chicken, salmon, ume (sour plum) and kombu (seaweed).
Onigiri should be eaten as soon as the nori is applied to the rice. Nori absorbs water from the rice and becomes very sticky and hard to eat with time (although some people prefer the sticky taste). Onigiri from Japanese convenience stores are specially wrapped to separate the nori and rice.
Onigiri have been eaten in Japan for over a thousand years and are still incredibly popular today. They are pretty cheap (usually 100 to 200 yen) and quite tasty. Japanese people make them at home and they are often found in restaurants such as Japanese pubs (izakaya).