Curved vanes have several bonus'. Primarily the curved vanes give a greater surface area which allows a faster heat transfer to cool the rotors faster. Curved vein rotors are rotational. Ensure that the rotor is on the correct side of your car.
Straight Vane rotors consist of two surfaces which are separated by vanes. The vent area between the braking surfaces allows air to flow between so the rotor can cool and maintain efficiency.
Solid Disc/Rotors - No vane
Solid have a single surface without vents and are more commonly used on very small kits, where space is limited and to replace OE equipment.