A cue sheet is a plain text file describing the contents of a CD or DVD  including the position of the tracks.
    It can also be used to describe the contents of a computer audio file.
    If you have problems tagging WAV, you can use a cue sheet as an  alternative.
    Some even prefer to rip a CD to a single file and use a cue sheet  instead of the more common file per track/tagging method.
    It can also be used to mark sections in long live recordings.
REM GENRE Classical
      REM DATE 2003
      REM DISCID CF10E50F
      REM COMMENT "ExactAudioCopy v0.99pb4"
      PERFORMER "Miklós Spányi & János Sebestyén"
      TITLE "Music  for two Organs from the 18th &19th Centuries"
      FILE "Music for two Organs from the 18th-19th Centuries.flac" WAVE
      TRACK 01 AUDIO
      TITLE "Daniel Gottlieb Steibelt - Sonata in G major"
      PERFORMER "Miklós Spányi & János Sebestyén"
      INDEX 01 00:00:00
      TRACK 02 AUDIO
      TITLE "Ferdinando Bonazzi - Pastorale in E flat major - Andante"
      PERFORMER "Miklós Spányi & János Sebestyén"
      INDEX 01 03:45:57
    
The number of tags is rather limited.
Anything prefixed with REM is a comment but programs can use this to store additional information not supported by the standard.
If you use cue-sheets  you create a dependency.
    The name in the FILE tag  must match exactly the name in the file system.
    If you rename the  audio file, you have to edit the cue-sheet as well.
    Likewise if you move  the audio file, you have to move the cue-sheet as well.
    Sounds trivial but  media players might rename and move audio files and ignore the cue-sheet at the  same time.
See the software section for more details