Onigiri is very basic and Japanese food that Japanese often eat for lunch. It's a rice ball of sticky rice, which is wrapped into nori. Usually in center of the rice ball, there is umeboshi, a special pickled plum or something else very salty which kills the bacteria, preserving the rice.
Triangle shaped onigiris are very popular in Japan. You can find those from any convenience stores. Onigiri is good to eat even when it's cold.