World War I - a global war originating in Europe that lasted from July 28, 1914 to November 11, 1928. It is also sometimes called "The Great War."
Armistice Day - on the 11th hour of the 11th day on the 11th month, an agreement was signed between the Allied Countries and Germany to end World War I. THe day was then remembered as Armistice Day and celebrated veterans of WWI. After several other international wars, President Dwight D. Eisenhower signed a bill to change the name to Veterans Day to include all veterans.
Veteran - a person who has served in the military
Armistice - an agreement to end a war
Military - armed forces of a country. In the USA, that includes the Army, Navy, Marines, Air Force, etc.
Federal Holiday - a day designed by the federal government at the national level. On this day, federally operated businesses such as post offices and public schools are usually closed.