Unlike spin or bait rods, which cast the weight of a lure or bait, a fly rod casts the weight of the line. The line's mass loads the rod on the back-cast and unrolls it on the forward cast; the fly is just along for the ride.
Rods are sized in line weights: 3wt for small streams and small fish, 5wt for trout on most water, 8wt for big trout and bass, 10wt+ for saltwater. The 9-foot 5-weight is the universal trout rod.