Calimari is usually deep-fried squid. It is usually a bunch of little rings (they look like little onion rings). It's chewy in texture. Some places try to sneak you cuttlefish (which is similar but cheaper) instead of squid.
Calamari is basically friend octopus. However, it really doesn't taste fishy at all. It is very crispy at some restaurants. The inside part can be squisy, but it doesn't really taste like much. I recommend trying it!
Derived from squid, Calamari was traditionally a European dish that is now served all around the world in many variations. In the USA it's often fried in batter or breadcrumbs!