MP3 - The most common kind of digital music file. Short for MPEG-2 Audio Layer-3, a digital media standard designed by the Moving Picture Experts Group (MPEG).
People use MP3s for digital music because they require smaller files than those created using CD-quality audio. As a result of this, more MP3s can be stored in the same amount of space than CD-quality files. Though settings can cause this to vary, generally speaking, an MP3 takes up about 10% of the space of a CD-quality audio file.
How MP3s Work
MP3s achieve this space savings by compressing the data that makes up the file. This is done by removing some of audio from the original. For this reason, MP3 is called a lossy compression format (i.e., some data is lost during compression). Compressing songs into MP3s involves removing parts of the file that won't impact the listening experience, and thus allowing it to be smaller. However, because some data has been removed from the file, an MP3 doesn't sound identical to the CD-quality audio file it began as. This has caused some audiophiles to criticize MP3s as damaging the listening experience.
The audio quality of an MP3 is measured by its bit rate, rendered as kbps. The higher the bit rate, the better the sound of the MP3. Among the most common bit rates are 128 kps, 192 kbps, and 256 kbps.
There are two types of MP3: Constant Bit Rate (CBR) and Variable Bit Rate (VBR). Many modern MP3s use VBR, a method of making files small by encoding some parts of a song at a low bit rate (for instance, those with only one instrument can be simpler to compress) while others are encoded using higher bit rates.
iTunes & MP3s
While MP3 is perhaps the most popular digital audio format online, the iTunes Store does not use it, instead employing AAC, a similar type of file that often delivers better sound and smaller files.
You get MP3s from:
iPod/iPhone/iPad compatible: YesMore about MP3s at Wikipedia.