Map from September 2014
The U.S. Department of State handles summing up the risks and making recommendations for travelers seeking to visit or live in another country or region of the world. When the State Department issues a Travel Warning to a particular country or region they want you to understand the various risks associated with traveling to this area and to consider whether you should visit this country/region at all. "They use these examples of reasons for issuing a Travel Warning including unstable government, civil war, ongoing intense crime or violence, or frequent terrorist attacks." Furthermore, they state that they want you to consider strongly not going to them at all."
A Travel Advisory is shorter in duration than a Travel Warning. It is announced to inform travelers of short-term events that may pose a danger to US citizens. They cite "examples for issuing a Travel Advisory to include: an election season that is bound to have many strikes, demonstrations, or disturbances; a health alert like an outbreak of H1N1; or evidence of an elevated risk of terrorist attacks." Generally when these events have ceased, the alert is canceled.