Panino is Italian for sandwich, it literally means small bread.
Panini is something I first experienced in France, where it's made with a special semi-raw bread. I've never seen anything quite like it in Italy, though.
Panini and Panino are pretty much synonymous. Panini is just the anglicized word, although it seems the Italians don't generally grill them. I've had Panini/Panino in Italy, France, the UK, and Canada.
Ok, I'll give you a pass if you are from Italy, for example.
However, the story was about the Bay Area... and growing up in California I noticed a trend that exploded about 10 years ago.
Fancy, casual restaurants stopped selling sandwiches and "grilled-cheese",
upgraded the bread, and started calling them Panini.
The price was also increased by $5 dollars/per for a marginally better product.
Panini is something I first experienced in France, where it's made with a special semi-raw bread. I've never seen anything quite like it in Italy, though.