Piloncillo is an unrefined sugar that is commonly used in Mexican cooking. The sugar has been around for at least 500 years, and was being made before the Spanish came to Mexico around 1500. It is made when sugar canes are crushed, the juice is collected and boiled then poured into molds, where it hardens into blocks. The fact that it comes in block form is one of the reasons why white and brown sugars are more commonly used, even in Mexican cooking, than piloncillo once was. To use it, it must be grated or chiseled off the main block - a process which is well worth the resulting flavor boost in food to some, but too time consuming for others to bother.
Unlike white sugar, which is flat and one-dimensional in its sweet flavor, piloncillo is smoky, caramely and earthy. It has far more flavor than brown sugar, which is generally just white sugar with a small amount of molasses added back to it. It can be uses in moles and other sauces, as well as to simple sweeten coffee or top off buttery toast. In most applications, it must be melted down to be incorporated into recipes. You can find piloncillo in many Mexican or Latin American grocery stores or order it online.