Webster 1913's writeup is a bit dated, but we can perhaps forgive him, since he wrote it long before the theory of plate tectonics developed, indeed before Alfred Wegener even proposed his theory of continental drift. Today, we understand that continents are built out of tiny pieces, one bit at a time.
A "terrane" is a unit of the Earth's crust having a common geological history that is distinct from the area around it. Terranes are the little pieces of crust that accrete onto continental cratons to make larger continents -- volcanic island arcs, chunks rifted off other continents, sediments from seabeds scraped up by colliding continents, fragments left behind when continents recently joined rift apart again, even pieces torn off a continent, carried along the coast, and stuck back onto the same continent somewhere else. The boundaries of terranes are usualy faults and suture lines.
"Superterranes" are terranes that are composed of other terranes that act as a unit after they have accreted together. So the identification of terranes is really a hierarchy that culminates in the independent ones, Earth's tectonic plates.
Terrane theory developed in the 1980's to explain the complicated geology of northwestern North America. The North American Plate's subduction of the Farallon Plate, and its later lateral transform with the Pacific Plate, caused any number of tiny pieces to be welded onto the western edge of the Canadian Shield. The Canadian Rockies and everything west to the Pacific Ocean are composed of five distinct belts of terranes, each with a distinct age.
Since then, geologists have been able to extend this idea over all of the Earth's continents. Large cratons such as the Canadian Shield were also constructed in this fashion over hundreds of millions of years, but they have been joined together for so long, people rarely try to identify the consiituents, and the Canadian Shield is probably the largest "terrane" recognized. Other areas that have recently1 been built from small terranes include China, Southern Europe, and the Middle East. Somewhat older is Eastern New England, as is the whole system of the Appalachian Mountains.
1"Recently" meaning within the last 200 million years or so.