Spiral cut gears have a circular shape to the teeth so on the ring gear the teeth start at the center but follow an arc to the outside of the gear. The other kind is straight cut gears where obviously the teeth are straight. Both are considered bevel gears it's the cut that's different.

Spiral cut gears can be quieter and some say stronger due to the spiral having more contact area. The downside is the spiral causes the gears to want to separate from each other increasing loading on the bearings and requiring more frequent checking and adjustment of the mesh (shimming) in RC vehicles.