People do call a part of the area that the Appalachian trail crosses "the Appalachians" -- e.g., in North Carolina, there is Appalachian State University, and people refer to the mountains as The Appalachians. But in other places, the mountains are called something else -- the Smokies, Berkshires, White Mountains, etc. It just depends on where you are. There are arguably "different" mountain ranges, but if you look at a topological map, it's basically one very long range of mountains. They were formed at essentially the same time by glaciers. They are smaller than a some other ranges around the country because they are older and have had longer to erode. Source: my 5th grade geology unit.