Fly fishing is using a fly rod and fly to catch fish, be it trolling or stripping/retrieving.

There are a lot of lake fly fishers who just troll, but most use a combination of both trolling and stripping. I prefer to strip for fish, but you have to get from one place to another so I troll.

Dry flies work best when the fish are actively feeding on/near the surface. You need to have feeding activity going on around you so that you can locate fish and cast in their direction.

If you are dialed in and actively fishing it will almost always out-produce trolling I have found. If you are casting to the wrong place, with the wrong fly with the wrong retrieve you will get out fished by the trollers almost every time.