What are the differences between the Canon 50mm 1.8 STM and the Canon 50mm 1.8 II lenses?

The Canon 50mm 1.8 STM lens has a newer design with smoother and quieter autofocus, while the Canon 50mm 1.8 II lens has a simpler autofocus system. The STM lens also has improved image quality and better build quality compared to the II lens.