Onigiri are to Japanese people as what pasta are to Italian people (maybe). They are stuffed rice balls wrapped with nori (seaweed), and the very basic take-out lunch for Japanese people. They come in round, or triangular, or in elongated shapes; their fillings are traditionally salty preserved food such as umeboshi (pickled plums), kelp, and salted salmon. However nowadays, there are almost no limits to what you can stuff into an onigiri, so people go wild with fried chicken, or mayonnaise and tuna, or whatever floats your taste boat.
Superficially, onigiris seem no different from sushi: rice, nori, and sometimes fish are the main components of the snack. However, they are actually different in many regards, especially to how rice is treated in each dish. For onigiri, plain rice is the main star, and fillings and salt are there to enhance the flavor of rice. Whereas for sushi, the fish is the main star, and the vinegar rice and soy sauce are preservatives (at least in the past), and are now accompaniments to the beautiful taste of fish.
Onigiris are easily made by hand, and many Japanese children eat home-made onigiris packed into their lunch boxes during school lunch. However, a faster alternative is onigiris offered in convenience stores and supermarkets. Traditionally, onigiris have short expiry dates, because seaweed wrapping often becomes wet and soggy after a short time of contact with the moist rice, and so they need to be eaten quickly while the seaweed is still crunchy. However, now, thanks for packing innovations, onigiri rice and seaweed become separated from each other before consumption through a plastic sheet, so the seaweed remains crispy even after a long day sitting on the shelf.
Onigiris have gained a bit of overseas recognition these days, with many shops opening overseas specializing in onigiris only, which is a refreshing alternative to the more expensive and stuffy atmosphere of sushi restaurants. The simple design and subtle delicious taste of onigiris is kind of all the nicest things of Japanese food wrapped into one small good thing.