The current list, developed by the US Department of State, includes: North Korea, Sudan, Syria, and Iran.
Being on the list can be devastating for the countries as it, in essence, excludes the country from trading or participating in commerce with the United States. This includes utilizing any of the US's banking resources or system.
It also means that US companies cannot sell weapons or a variety of other items to those countries and that US companies cannot buy from companies in those countries.