What is the difference between a P-51 Mustang and an A-36 Apache?
They were largely the same aircraft but the first line of the A-36 wiki sums it up nicely


  the ground-attack/dive bomber version of the North American P-51
  Mustang, from which it could be distinguished by the presence of
  rectangular, slatted dive brakes above and below the wings


later in the article they go on to list some more features 


  The contract for 500 A-36A aircraft fitted with bomb racks, dive
  brakes, and heavier-duty wing, was signed by Kelsey on 16 April
  1942,[6] even before the first flight of the first production P-51 in
  May 1942.


This website also lists the same mods as well as some other fun facts. 

This website lists two other features noteably the vent window which is not mentioned elsewhere. 


  ...and the installation of small vent windows in the windscreen side
  panels. In addition, two Browning M2 .50 caliber machine guns were
  mounted in each wing, and two more .50 cal. guns were mounted in the
  lower nose to fire through the propeller.