1. Low Condition (Green). This condition is declared when there is a low risk of terrorist attacks.
2. Guarded Condition (Blue). This condition is declared when there is a general risk of terrorist attacks.
3. Elevated Condition (Yellow). An Elevated Condition is declared when there is a significant risk of terrorist attacks.
4. High Condition (Orange).  A High Condition is declared when there is a high risk of terrorist attacks.
5. Severe Condition (Red).   A Severe Condition reflects a severe risk of terrorist attacks.

Go to: http://www.dhs.gov/dhspublic, and click on ‘Threat Advisory’ for additional information on the System Levels.