2014-09-20 : 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.

2014-09-20 : Once it triggers, prowess isn't connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.

2014-09-20 : Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.

2021-03-19 : An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.