What is a sparse file, and can they be identified?

Issue

• What is a sparse file, and can they be identified?
• There's an inconsistent between the values reported by commands such as ls and df.

• Found a file reporting a size larger than the space used in the filesystem where it is located. How can this be possible?

  # ls -l /templates/disk.img
  -rw------- 1 root root  40G Oct 18 10:05 /templates/disk.img
  
  # df -h /data
  Filesystem            Size  Used Avail Use% Mounted on
  /dev/vdb1              20G  9.8G  9.0G  53% /templates