Onigiri are Japanese rice balls and are often round or triangular in shape. They can be made of plain rice, or have a salty/sour filling inside. They are also often wrapped in nori (seaweed).
Onigiri is quite simple to make at home. Cook some short-grained rice (using a rice cooker is easiest). You can make the onigiri with plain rice, or sushi rice. I like to make onigiri with sushi rice as it adds more flavour.
For sushi rice, put the rice in a large container and stir in some sushi vinegar (available at most supermarkets or Asian supermarkets these days). If you must, you can also make your own sushi vinegar by mixing rice vinegar, sugar and salt together. Continue to stir the rice and fan it a little as you stir to cool it down slightly. Taste the rice and add more sushi vinegar until you are happy with the amount.
The easiest way to make onigiri is with an onigiri mould. You can find a cheap plastic mould quite easily these days in Japanese supply stores.
Or else, tear off a piece of strong plastic wrap (around 20cm in length) and put it on the palm of one hand (with your palm bent in a cup-shape). Scoop some rice onto the plastic in your palm, add some filling in the middle, and scoop more rice on top to cover the filling. Use a tablespoon or teaspoon to measure how much rice you add so your onigiri are similar in size. Close your palm, gather up the ends of the plastic wrap with your other hand, and twist the wrap together. Squeeze the rice tightly together to form your onigiri, and shape it according to whatever shape you like (round, triangular, etc).
Update: See my post on How To Make a Monokuro Boo Bento for a pictorial guide on making and shaping onigiri this way (See Steps 1 - 4).
Make sure the rice is warm, or the onigiri will fall apart. Re-heat the rice in the rice cooker or microwave if it cools down halfway.
Wrap the onigiri in nori (seaweed) just before eating. You can also wrap them beforehand, if you don't mind soggy nori.
Some common onigiri fillings are plum, salted salmon, tuna, teriyaki chicken.