Teeth with a special curved side surface work better than teeth with straight sides.
Gears with this curvature are called involute gears.

The teeth may in width (axial) be straight or slightly oblique to the axis (screw gear). The sloping give more, longer and better contact, so the course is accompanied with less vibration. Helical gear is used e.g. in gearboxes.

Conical (cone -shaped) gears are used to transfer rotating movement between axes which are not parallel.