A Pro CD-R is a is a professionally duplicated (burned) disc with clean printing. The audio data has been burned into the disc with a laser (this is “duplication”). The P stands for “professionally-manufactured."
A CD refers to professionally replicated discs, made from a glass master. This means that the disc has had information permanently stamped into the reflective aluminum layer (this is “replication”). CDs can only be professionally manufactured.
All distributors accept professionally replicated CDs; many accept Pro CD-Rs.
Because this decision is up to each wholesale distributor and each individual customer, it is important to clearly and correctly identify the CD format for your album when signing up: CD, Pro CD-R, DVD or Vinyl.