HISPANIOLA is the name of an island in the Caribbean sea that includes the two countries HAITI and THE DOMINICAN REPUBLIC. Please see the attached wiki fike that gives you more detail about the island.
SANTO DOMINGO is the capital city of The Dominican Republic, one of the two countries on the island of Hispaniola. Again please see the wiki file below for greater detail.
"....Santo Domingo, known officially as Santo Domingo de Guzmán, is the capital and largest city in the Dominican Republic"