What is the difference between a squid an octopus?

• When hunting, some octopuses inject poison into the prey, causing paralysis.
• Octopuses range in size from 1 cm to more than 5 m in length, while squids range from 1 cm to 20 m.
• Octopuses live in dens on the sea floor. Squids live in the open oceans.
• Both octopuses and squids are related to snails and other molluscs. Unlike snails, octopuses no longer have any remnant of a shell. Squids possess a stiff structure known as a pen, that acts as a flexible backbone.
• Octopuses are solitary animals that live alone in dens.

Some squids live in large schools, others are solitary. Some squids school when young and become solitary later in life.

• Squid have two tentacles and eight arms, and octopuses just have eight arms.

Edit: This list seems to be all wrong (except the very last point, which I need not discuss), thusly highly misleading.

• All squids and octopuses have a venom gland and venomous bite

  Read more: Is_the_common_squid_poisonous

• The giant squid (genus: Architeuthis) is a deep-ocean dwelling squid in the family Architeuthidae, represented by as many as eight species. (wikipedia)
• Deep/open. I'll let you have this one because I can't fact-check it in 2 minutes (remember, I found this question because I'm looking for the answers myself)
• This could be right. I remember watching a highly flexible octopus (on a show about problem solving abilities in animals). Squids seem to be less prone to changing shape.
• I've seen schools of squids. This one could be right too.