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Digital audio comes in many different file formats. Each format has its own advantages, or a special niche. Some offer smaller file size, and let you fit more sound on the same auido player or disk. Some formats, such as MP3, are great for their compatibility, and will work with most hardware and software out there. Some online services will use their own proprietary file format.

Files in the same format can also be of different quality and size. In audio file formats sample rate, bit rate, and number of chanels are factors that can be changed to make the file smaller/worse quality, or bigger/better quality. Sample rate is the number of samples per second used to describe audio, and is measured in Hz or KHz. Bit rate is the amount of memory used to describe a second of audio, and is measured in kilobites per second, or Kb/s. Number of chanels can be Mono, Stereo, and sometimes more advanced options such as 5.1 Suround.
If these factors are decreased, the file becomes smaller, and the quality gets worse. If they are increased, the file size gets bigger, but you can not get increased quality beyond what the original provides. Each time a file is changed from one format to another, it loses some quality.

Metadata or tags is the name for the text and pictures that is displayed on your audio players screen when an audio file is playing. Each file format has its own scheme for tagging files.
Metadata will let you see such things as Title, Artist, Album, Lyrics, Album Art, File Format, Bit Rate. Programs that let you convert files from one format to another, will usualy let you edit metadata tags, at least for popular formats.
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