Getting a grip

Why is an octopus called an octopus if it has ten-tacles? Yes, now I know that it has to do with "tentacles" - as derived from the Latin tentaculum which means to feel/probe. But what is the difference between a tentacle and an arm ... especially if octopuses, squid and cuttlefish all have 8 - but cuttlefish and squid have 2 extra but different "tentacles". According to Dr James Wood: tentacles are longer then arms and they usually only have suckers at their tips.