What are the main differences between traditional fishing with a spinning rod or baitcasting rod and fly fishing?

The main differences between the two methods of fishing are the types of lures that are used and in the way the lure is delivered by a cast of the rod.  In traditional or spin fishing, the lure itself is weighted and this weight provides the leverage necessary to cast out into the water.  In contrast, fly fishing the lures are typically called flies and are very light.  Because they are so light, a heavier line is used and the weight of the line is what allows the fly to be cast out and delivered to the water.  Spin fishing lures are often measured in ounces of weight and are often made of metal or plastic.  They can range in size and design depending on the target species of fish.  Fly fishing flies are typically very lightweight hairs, threads, or other materials tied to a hook.  The fly fishing line weights are measured in grains.