The best way to understand octopus and squid is to go to the aquarium and look at them. You can see them catching their food and eating it. In some places you might even be able to touch one!
Here is a site that will show you an aquarium near you.
You might also be interested in eating squid or octopus. In restaurants, squid are usually called "calamari". They are a little chewy but very tasty. Squid is a pretty sustainable seafood, and good for you too.
To find out more about cephalopods, check out these books from Amazon.com or from your library: