Music - Audio CD:
An Audio CD (one that plays on your standard stereo, and contains music separated by tracks) can hold about 80 minutes worth of audio programming. (79 minutes, 59 seconds to be exact)
Data - CD-ROM:
A CD-ROM can also hold virtually any type of DATA you would like. A typical CD can hold up to 700 MBs worth of DATA. This DATA could be PDFs, MP3s, Microsoft Word Documents, Quicktime Movies, pretty much anything as long as you do not exceed the 700 MB limit.
KEEP IN MIND: Unlike your typical audio CD, that should be playable by anyone that has a stereo or computer, the end recipient must have the program capable of opening the data on the CD-ROM.
Simply putting your audio files (MP3, M4A, etc) onto a CD without authoring the disc to the proper format will result in inconsistent playback across different devices.