In the simplest terms, “Longboard” are generally in the 36”- 60”range. Longboards were specifically designed to be ridden down steep hills, as their “carve” (or turning ability) is a lot tighter and more agile than regular-sized skateboards. Imagine slalom skating competitions where riders carve around orange traffic cones at high speeds. This is the “heart” of Longboarding.
"Shortboards" are generally in a range of 24”-35” and are designed for street and vert tricks.