Onigiri or omusubi are exactly same. The name nigiri came from the verb nigiri in Japanese which means grab a hand, or grab by the hand. Omusubi came from the verb musubu which means to connect something. Both words combine to means to make one thing with many things. Therefore the meaning of onigiri can be to make a ball with each rice grain by hand.
There are many variations and it’s endless. The traditional ones are with grilled salmon (my favourite!) and umeboshi (Japanese fermented plum pickle). This shio-musubi has no filling, only salt. Simple is best! Because the only added flavour is salt, it goes well any kind of meal. Choose your favourite salt, and enjoy the simplicity.