Cephalopods are a diverse group, which is like saying Godzilla must leave a lot of dino dung on the Japanese landscape: the description is wholly inadequate.
Squid in particular fascinate a lot of humans. The giant squid and colossal squid are of course the main subjects of popular culture and bad movies. (There are a couple of squid-related novels out: Ryan Lockwood's Below is pretty good even if, like Peter Benchley's giant squid in Beast, his Humboldt squid are a little too smart and way too emotional. Below works very well because the Humboldt squid, 2-3 meters long and a voracious, pack-hunting, communicating animal, really could be scary as hell if it ever collectively decided to find a new food source. Scarier than Benchley's 100-foot giant squid, in fact, because giant squid may be dangerous but they are rare compared to the Humboldt. And Lockwood, unlike many "creature" novelists, spends plenty of time developing realistic human characters. So kudos to him. Greig Beck's 2010 novel Beneath the Dark Ice is well-reviewed, but I haven't read it yet.)
We catch squid, eat them, dissect them, and put them (the smaller ones, anyway) in tanks, but their world isn't our world.
While it's estimated there are 300-ish species in the order Teuthida, new ones are described every year, and we learn many new things about species we have classified.
Take, for example, Grimalditeuthis bonplandi, a translucent species about 15 cm long that lives in the Pacific. It was described from trawled (dead) specimens, but it's been captured on video for the first time, and it has a surprising habit.
While we think of all squid as having eight shorter arms and two long food-grasping tentacles, there are a number of variations on this plan. The vampire squid (okay, maybe not technically a squid, depending on who you ask), has two "luring" tentacles, very long, thin spaghetti-like appendages with luminescent tips, used to entice prey close enough to be grabbed. G. bonpladi has only one such tentacle, with no luminescence, and lets it wave around in the current to either stimulate tiny light-bearing invertebrates to just look like a food object far tinier than the squid itself. When something the right size move in to grab the lure, the squid jets in and uses the stronger arms to attack. Or so we might presume. It hasn't been caught in the act of feeding yet.
Squid are a critical part of the ocean ecosystem, and who knows what the ocean would look like if we wiped out their primary nemesis, the sperm whale? (Sharks and other fish also eat squid, but we're wiping them out as I write this.) We have a lot to learn....