Heraldry arose in the 12th century around the time of the Crusades.  In battle, a knight dressed in armor from head to foot would be barely recognizable by friend or enemy so a new method of identification became necessary.  This resulted in special marking being painted on the knight's shield and on the coat he wore over his armor, hence the term Coat of Arms.  The shield was generally made of a wooden frame across which a leather hide was stretched.

The rim and center were studded and metal bands were painted or decorated with each knight using his specific metal or color.  A son would inherit the markings of his father, carrying them into battle with pride.  After battle, the knight returned to his castle or manor and hung his shield and helmet on the wall.

The colorful tournaments or jousting competitions of the Middle Ages helped greatly in the development of heraldry.  However, when jousting was no longer in fashion, the individual markings used by knights began to appear on seals, in stone, on stained glass and on objects of value, all of which identified the owner.  This was very important in an age when many people could not read or write.

Heraldry has its own special language which comes from Norman French, although the there are also works of Turkish origin, first introduced by the Crusaders.  While the Norman French are rightly credited with the introduction of heraldry to Britain and Ireland, the charges found on the arms of native Irish families are often pre Christian or of Druidic origin.

Descriptions of Coats of Arms known as Blazons were often written in the native language of the individual country.

The Coat of Arms is itself described in the Blazon and in most descriptions, the first word mentioned is the color of the shield.  Above the Blazon is found the knights helmet on which is placed the Crest.  The markings on the shield are referred to as "charges" or "ordinaries" while the right side is referred to as "dexter" and the left side as the "sinister".

Source - "Heraldica"