I got this information from an Adaptec mailing list. All compact discs have errors on them. Some of them have a lot of errors. Even with the errors, however, you can still read a CD accurately.
Logical errors are errors in the structure of the user data and control data on the disc.
Physical errors are actual microscopic physical artifacts on the disc produced during the production or recording process, or defects introduced after production, like scratches or foreign materials on the surface of the disc. Whenever we talk about disc quality, we are talking about the interaction of the disc with the mechanism that it is being played on. These errors can be caused by scratches or dirt on the disc, or can be a result of a poorly manufactured or recorded disc. It does this by using other data on the disc that exists for the specific purpose of correcting errors. CIRC applies two levels of error correction, called known C1 and C2.