22 Jun 2015
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.
22 Jun 2015
Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.
22 Jun 2015
Once it triggers, prowess isn’t connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.