Onigiri is everywhere. Basically, it's a ball of rice with something inside; most onigiri includes fish or other seafood of some sort. It's also usually wrapped in a piece of nori, which is dried seaweed. I've been trying to develop a taste for nori, but there's something about the scent that, to me, is off-putting. Most people (Japanese or western) don't have a problem with it.
Japanese people usually eat onigiri as a snack. The large amount of rice is filling, and it's easy and quick to grab at a convenience store and eat before catching a train. I've had it as a light meal though, before or after a larger meal. And two of them makes for a very tasty and filling breakfast.
The nori is like bread in a sandwich. It holds it all together and keeps the hands clean. The rice is sticky, and will generally stay together, though. There are a few varieties of onigiri that come without the nori, and I've been trying them.
There are plenty of nori-wrapped variations too, and I've also tried an oyster onigiri (not wrapped in nori) which was decent. My favorite is the fried rice now, though hopefully in a month or so I'll be chowing down on ones with nori.