What is the difference between an artifact and a creature?
An artifact is simply a card type, like "land", "creature", "instant" etc. After you cast an artifact card, it just sits there on the battlefield and only does whatever its own text says it does.

A creature is also a type of card, but creatures are different than other card types in that they can attack and block.

An "artifact creature" is just an object that is both an artifact and a creature. So Renegade Freighter, as an artifact, cannot attack or block. But because it is also a vehicle, it has a "crew" ability that lets it become an artifact creature, so that you can attack and block with it.