NFPA 704 is a standard maintained by the U.S.

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

-based National Fire Protection Association

National Fire Protection Association

The National Fire Protection Association is a United States trade association that creates and maintains private, copywrited, standards and codes for usage and adoption by local governments...

. It defines the colloquial "fire diamond" used by emergency personnel to quickly and easily identify the risks posed by nearby hazardous materials. This is necessary to help determine what, if any, special equipment should be used, procedures followed, or precautions taken during the first moments of an emergency response.
The four divisions are typically color-coded, with blue indicating level of health

Health

Health is the level of functional or metabolic efficiency of a living being. In humans, it is the general condition of a person's mind, body and spirit, usually meaning to be free from illness, injury or pain...

 hazard

Hazard

A hazard is a situation that poses a level of threat to life, health, property, or environment. Most hazards are dormant or potential, with only a theoretical risk of harm; however, once a hazard becomes "active", it can create an emergency situation. A hazard does not exist when it is not...

, red indicating flammability, yellow (chemical) reactivity, and white containing special codes for unique hazards.