Please, a fairly laymans explanation of the differences and uses of schedule 40 & 80 pvc and cpvc.

They are the same material but different thickness. This should help. PVC and CPVC Pipes - Schedule 40 & 80

Bob gave you a detailed answer. Let's see if I can "simple" it out.
PVC is for drainage and cold water lines. CVPC is for hot water lines.
Schedule #20 is used in lighter applications with not much pressure.
Schedule #40 is the most used in water piping and drainage.
Schedule #80 is used in applications that require a heavier stronger pipe such as a commercial contract would call for. Hope this answers your question and thank you for rating my answer. Tom