In heraldry the various component parts of what is popularly called a coat of arms, are more properly collectively known as 'an achievement'. The foundation of everything is the shield to which may be added the helmet, mantling, wreath, crest, motto, coronet or cap of rank, robe of estate, compartment and supporters. In addition, there may also be special badges of rank or office as can be seen in the arms of the Dukes of Argyll or the Earls of Erroll. The diagram below explains the Arms of Keith, Earl Marischal.
Source: Keith & Kin, 4th Qtr 1992 & 1st Qtr, 1993