What is the difference between a coat of arms and shield?

A shield is a physical item used for a physical defense of a person. A coat of arms is a symbol representative of a person, family, or a country. However, in many languages, the word for shield and coat of arms are the same (such as Spanish - escudo) and most nobles would emblazon their coat of arms on the surface of their shields as a form of personalization of their military gear.