There were also the audio CDs that had data on them. Audio CD players would just play the audio, but a CD-ROM could access both. Some had apps that were games that would play the audio portion for the game.
The Mac version of the original Descent was like this too, with a great redbook audio soundtrack. The game wasn't locked to the original disc though, you could pop out the CD in the middle of the game and replace it with any other audio CD and it'd play that just as well.
If you want to know about the different types of CDs, you'll want to know about the various colors: https://en.wikipedia.org/wiki/Rainbow_Books