What's the difference between schedule 40 and 80 PVC?

Schedule 40 is typical PVC for routine plumbing and will handle up to 140 psi (@ 73 degF). Schedule 80 has a thicker wall and will handle up to 200 psi (@ 73 degF). Schedule 80 is seldom used for these applications therefore it's usually only used because the code might require it.