What is the difference between wheat barley maize?

Wheat has a more broader and "coarser" seed head than barley, and the tufts/bristles (called awns) are shorter in wheat than barley. Corn or Maize is a much larger cereal grass, with the seeds on a cob enclosed by a sheath. The awns are very floppy and finer than either wheat or corn. Corn can grow up to 10 feet in height whereas barley and wheat only grow up to 4 feet in height.

As far as deciphering individual grains, wheat is darker in colouration than barley. Corn is much different shaped than wheat or barley, having a rounded head as opposed to the sharp oval/diamond shape that wheat and barley have.