Above are the the traditional hand-formed onigiri shapes: triangle, flattened round, and cylinder or tawara. Tawara is the shape of the traditional straw bale that was for storing and transporting rice.
Type 1: The plain, unadorned, unfilled onigiri
This is a plain ball of rice, lightly salted on the outside. There is no filling, nor any cover. If one has excellent quality rice, such as top class shinmai (new rice from the current harvest) and wants to savor the pure flavor of the rice, this is the one to have.
A plain onigiri like this is the essence of Japanese food to me: rice, and salt. Rice was so important that the wealth of lords used to be measured in how much rice their lands produced, and salt is used extensively in Shinto rituals even now, to purify and sanctify. The onigiri as religious icon? Why not?
Type 2: The filled, nori covered onigiri
This is the most popular kind of onigiri, with a small amount of salty filling in the inside covered with plain rice, which is covered partly or fully with nori. Depending on if you like your nori crispy or a bit soft and moist, you either carry the nori separately and wrap it around the rice when eating, or put it on the rice when making (and when the rice is still warm). Since it’s like thin paper, it can be cut easily with scissors, and is used quite a lot for decorative ‘cute’ bento.
Type 3: The sprinkled onigiri
This is a filled or unfilled onigiri that is sprinkled on the outside with something. Sesame seeds, gomashio (sesame seeds mixed with salt), or furikake (mixed savory sprinkles - there are many various flavors) are commonly used. The one on the left is sprinkled with gomashio, and the one on the right with two colors of yukari (dried shiso leaf powder).
Type 4: The mixed-rice onigiri
For this type of onigiri, the rice is first mixed with something, then formed into a ball. The example above on the left is mixed with green peas, and the one on the right is mixed with homemade furikake made from radish leaves and bonito flakes (recipe ). Since the rice is flavored, this type usually doesn’t have a filling, and is often not covered to show the rice off (or just has a minimal nori strip). Anything can be mixed into the rice like this as long as it’s not too moist or oily, which will make the rice grains fall apart.