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.
While dulce de leche is becoming increasingly popular, it is still not widely available. As I mentioned before, it is possible to make your own, but if you’re not inclined to do so, buying it is also a good option. You will be able to find it at Mexican grocers, if there are any in your area, and at specialty foods stores, like Williams-Sonoma. Some mainstream grocery stores may carry it, but at the moment, most seem not to. 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.