To prevent any confusion, let me first state that the terms onigiri and omusubi are completely interchangeable. The names are simply regional, like how some people here use the terms "hot dog" and "weiner" interchangeably. Also, onigiri/omusubi are NOT a form of sushi. The rice used for onigiri is plain rice, lightly salted on the outside, while sushi uses sushi rice, flavored with vinegar, sugar and salt.
Onigiri is a classic Japanese snack, dating back to the 11th century, available in pretty much any konbini (convenience store). Perfect for travel; cooked rice is formed into a triangle shape and wrapped with nori (dried, toasted seaweed). Often, the onigiri has a salty or sour filling, like umeboshi (pickled plum), shiozake (salted salmon), katsuobushi (dried, shredded bonito) or takuan (pickled daikon radish); this was used as a natural preservative.
Nowadays, onigiri serves as a snack or light meal, and is as ubiquitous in a Japanese bento as a sandwich would be here in a child's lunchbox.