What's the Difference Between CD-ROMs, CD-Rs, and CD-RWs?
CD-ROM
When CDs were still used, the CD you typically bought with software or games and stored in it would be the CD-ROM. ROM stands for Read-Only Memory. These CDs had data in it that your computer could read if you had a CD drive, but you couldn't modify the data in the disk. This was physically impossible due to the properties of the disk itself. The only thing you could do was copy the files to your hard disk and edit them there, but the files in the disk were read-only.
[The CD-ROM] is most commonly used to distribute software and video games, and is a read-only format (CD-R and CD-RW were introduced later as writeable formats)
https://obsoletemedia.org/cd-rom/ (accessed 2025-01-15)
CD-R
If you wanted to record something yourself, you had to buy a CD-R. The R stands for Recordable. These would be "blank" CDs that you could use to save your own files. However, there were caveats.
The first caveat is that this was only possible if you had a CD recorder. That is, the CD drive that PCs had used to be able to read data from CD-ROMs, but weren't capable of writing data to CD-Rs. Eventually, CD-Rs became more popular, and the CD drives started being able to record them. Or at least mine was able to to burn Windows using Nero after much struggle.
Subject: [1-3] Can I create new audio and data CDs?
(2001/11/09)[...]
Writing to CD-Rs and CD-RWs requires a CD recorder. You can't write CDs with an ordinary CD-ROM drive.
https://www.cdrfaq.org/faq01.html#S1-3 (accessed 2025-01-15)
The second caveat is that although CD-Rs aren't read-only (they aren't CD-ROMs), they are WORM memory (Write Once Read Many), which means that you could record data using a recorder, but after you recorded it, you wouldn't be able to modify it. In other words, it's like you bought a magic blank CD-ROM that magically wasn't read-only while it was blank, but became read-only after you saved anything in it. Burning the CD with a laser physically altered its physical properties, and they couldn't be altered twice. That's why they worked the way they did.
A CD recorder writes data to a CD-R disc by pulsing its laser to heat areas of the organic dye layer. The writing process does not produce indentations (pits) – instead, the heat permanently changes the optical properties of the dye, changing the reflectivity of those areas. Various dyes have been used over the years, with cyanine being the earliest and less stable and phthalocyanine being the most stable).
https://obsoletemedia.org/cd-r/ (accessed 2025-01-15)
If, while burning the CD, an error occurred and the CD-R finished burning unsuccessfully, e.g. a file that should have been saved wasn't, or was saved wrong, corrupting the data, there were no do-overs. The CD-R was ruined. You had to throw it in the trash and start over with a different, blank CD-R.
CD-RW
The CD-RW was a ReWritable CD format. This was like a CD-R, but the data could be overwritten just like the data in your hard disk.
The advantage of CD-R over other types of optical media is that you can use the discs with a standard CD player. The disadvantage is that you can't reuse a disc.
A related technology called CD-Rewritable (CD-RW) allows you to erase discs and reuse them, but the CD-RW media doesn't work in all players.
https://www.cdrfaq.org/faq01.html#S1-1 (accessed 2025-01-15)