Audio CD versus Data/mp3 CD

Audio CD
While some people are very knowledge about the different types of CD and DVD, some others are still very confused on the different formats. Although more and more people want to enjoy the MP3 technology by making their own MP3 discs for their cars, portable mp3 players, etc... many of them are not sure how to create a MP3 CD and how it is different comparing to a regular audio CD.
To make it simple, an audio CD (Compact Disc Digital Audio) is one that you buy from stores and contains only audio tracks. Audio CDs are compatible with most of current CD-drives, including your CD/DVD drive on your computer, and your car CD player. For little more details, this standard of Audio CDs is called the "Red Book", which was first released in 1980 by Philips and Sony.
The audio tracks are uncompressed digital data (essentially WAV). This explains why you can't fit 100 songs in an audio CD like an MP3 CD because the size of each track is big. Try to plug an audio CD into your computer CD/DVD drive, what you'd see are some files with the names similar to "track01.cda". These are audio tracks, and you can't just open them with your computer like any other computer file. A CDA file simply points to the location of the audio track on the CD. Also, you cannot copy these CDA files to another location with your computer. Why? because they contain no audio data, they are just the "pointer" files. The actual audio data is stored on the CD sectors and cannot be viewable on your computers.
An audio CD normally can hold up to 74 or 80 minutes of audio. So when you create an audio CD from some MP3 files, no matter how big or small your mp3 files are, the CD can only fit in up to 80 minutes of audio.
- Pro: Compatibility. Audio CDs can be played on most CD players/drives.
- Con: Size limit. You can't fit many songs into an audio CD.
Audio Spec

• The frequency response: from 20 Hz to 20 kHz.
  
• Bit rate = 44100 samples/s × 16 bit/sample × 2 channels = 1411.2 kbit/s (more than 10 MB per minute)
• Sample values: range from -32768 to +32767.
• On the disc, the data are stored in sectors of 2352 bytes each, read at 75 sectors/s.

MP3 CD
MP3 (or "MPEG-1 Audio Layer 3") is an encoding format for audio data. Not to be confused with "MPEG-3", which is a video standard. MP3 is an audio-specific standard that was introduced in 1991.
Late 1990s, the internet users started to encode more and more audio files using the MP3 format and spread them over the internet. This lead to the creations of many MP3 player software such as WinAmp, Nullsoft, etc... The MP3 format became the most popular audio format for computer because of the advantages of it.
An MP3 file is a compressed audio file. This brings the biggest advantage of the MP3 format: the small size. Although the size of each MP3 file is vary, depending on many factors, an average size of an MP3 file for a music track doesn't go more than 7 MB.
MP3 files are just like any other computer files. You can move them from place to place on your computer, or you can open them using different application software. Once it comes to the idea of making a CD out of these files, it can be a little confusing. Why? apparently, many people think that they're creating an audio CD instead, just because they are expecting a "CD with audio" at the end. In fact, an MP3 CD is not like an audio CD but like an any other computer discs: it contains data files, not audio tracks.
So what makes it a big deal? Many CD-copy software have different selections for "Audio" and "Data". The "Audio" option is for creating a standard CD with audio tracks, while the "Data" option can be used to copy either computer data or MP3 files. Remember MP3 files are just like computer data files!
Now the term "bitrate" might come up while you're playing with MP3, so what is it? Bitrate is the number of bits that are processed per unit of time. Simpler, it defines the compress ratio on the MP3 files. A lower bit-rate would give you a smaller MP3 file, but also.. less quality.
- Pro: Size! A CD with MP3 files can hold up to 700 MB of data/mp3 files. So with an average of 4-5 MB each MP3, you can store up to 160 songs on a CD, that's about 10 times of an Audio CD.
- Con: Compatibility. Although more and more players are compatible with MP3 standard, there are still some that only play audio CDs.