Scientifically speaking, a squid is not a fish, but a mollusk. Mollusks are soft-bodied creatures that have gills in order to breath through.
Squid live in various bodies of water throughout the world. There are around 300 different recognized species as of 2014. Squid vary in size greatly and while some only grow to 25 inches in length, there are certain species that can grow to astonishing lengths of around 40 feet. Due to their soft bodies, squid are preyed upon by many other sea creatures, including larger fish. Squid are also commercially fished as they are considered delicacy in some cultures.