Basically, a writeable CD, or a CDR, contains a dye that is aligned with a laser to construct the data layer of the CD.
A silver pressed CD actually contains a physically stamped data layer, making it much more durable. It also means that it's an original and has not been through many generations of potentially flawed extracting and re-burning, which is why we prefer to have seeds from original silver pressed CDs seeded if possible.
Take a look at your original discs, and notice how they are silver compared to the green or purple or blue of the dye of your CDR.
If you really want some insane detail on the subject, here's an awesome link to everything you ever wanted to know about audio CDs:
http://www.cdrfaq.org/
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