Fixed rotors are normally cut out of one piece of steel and feature an array of cut outs to reduce weight, improve the braking surface, shed mud and dirt and to aid cooling.
A floating rotor is normally based on a steel outer section (braking surface) which is riveted to a aluminium inner (spider), which bolts to the hub. The rivets allow the two materials to deal with extreme heat (due to hard braking) independently from each other. This helps prevent the rotor from warping under heavy braking. The combination of the steel braking surface and the aluminium carrier can also save weight.
Fixed rotors are normally less expensive than floating rotors due to the usage of less expensive materials and a simpler production process.