Metalcraft is an ability word.
It is activated only when you control three or more artifacts. So in the example of Screeching Silcaw, this is how metalcraft works:
Metalcraft - When Screeching Silcaw deals combat damage to a player, if you control three or more artifacts, that player puts the top four cards of his or her library into his or her graveyard.
So this means that if you control three or more artifacts, the Screeching Silcaw gets a new ability. But, like landfall, this is not the only thing that can happen. It's possible that a metalcraft card can get +2/+2, or get trample, or any other ability you could normally get. The only rule of metalcraft is that you must have three or more artifacts for the ability to be activated. Therefore, metalcraft works best in an artifact deck.