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

Issue

• The file size is inconsistent between ls and df for a file.

• This file reports a large size (128Gb) but the filesystem isn't even that big. How is this possible?

  # ls -lah datafile
  -rw------- 1 root root  137438953440 May 28 10:05 datafile
  
  # df -h
  Filesystem            Size  Used Avail Use% Mounted on
  /dev/sda               20G  9.8G  9.0G  53% /