An artifact is simply a card type, like "land", "creature", "instant" etc. It has no special rules associated with it by virtue of being an artifact. 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 it has special rules associated with it because it's a creature. The main ones being that creatures 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.