Dulce de leche is a thick, milk-based caramel sauce. It is probably most popular in South American Spanish and Portuguese-speaking countries, but variations exist in other countries and cultures as well. The most simple caramel sauce starts with sugar, and depending on the variation you have in mind, ingredients like cream and butter are added to the hot sugar to form caramel. With dulce de leche, milk and sugar are cooked together to make a caramel, a process that takes quite a long time but results in an incredibly rich-tasting sauce with a strong milky flavor behind its sweetness. It is similar to sweetened condensed milk (and can be made from it, too), although dulce de leche is thicker and much more caramelized (darker in color and flavor) than sweetened condensed milk alone.
As dulce de leche is becoming increasingly popular, it is also becoming much more widely available, where once it was only easily found in specialty stores. Just about every regular grocery store in my area stocks it, usually in the baking aisle near the sweetened condensed milk. If you can’t find it in your local store, check out a Mexican grocery store or a specialty foods store, like Williams-Sonoma. You can also find different brands online, if you do a quick search. Try out a few different brands if you are ordering, since dulce de leche, like other types of caramel, can taste different from brand to brand.