2014-11-24 : Any spell you cast that doesn’t have the type creature will cause prowess to trigger. If a spell has multiple types, and one of those types is creature (such as an artifact creature), casting it won’t cause prowess to trigger. Playing a land also won’t cause prowess to trigger.

2014-11-24 : Once it triggers, prowess isn’t connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.

2014-11-24 : Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.

2014-11-24 : Prowess triggers only once for any spell, even if that spell has multiple types.