Dulce de leche is basically caramelized condensed milk. Dulce de leche can be pour it on top of fruit and used in many dessert recipes. It's not uncommon for this delicious treat to be spread on top of pancakes or plain toast.
The sundae was invented when soda jerks in the late bowed to criticism from religious leaders for serving "sinfully" rich ice cream sodas on Sundays. They started serving the ice cream and syrups without soda water and called it a "sundae.