Biodiesel is an alternative fuel made from vegetable oils or animal fats through a chemical process called transesterification. It produces significantly lower emissions than petroleum diesel and can be used in place of or blended with conventional diesel in most vehicles. The document discusses biodiesel availability in the western United States, showing locations where it can be purchased and analyzing the driving range of different vehicles if fueled solely on biodiesel. A map displays biodiesel coverage across the region, determining it is possible to drive from Mexico to Canada using only biodiesel stations, though certain vehicles may have challenges with availability.