Sweet vermouth is kind of like the acoustic guitar: it didn't require a modifier until a newer version came along. The products we know today by groupings such as sweet, Italian, and red vermouth originated in and around the city of Turin (aka Torino) sometime during the 1700s. Soon after, drier-style riffs on these products began to surface, prompting a need for finer classification.
Now we've reached the point, helped by renewed interest in old cocktails, where there's seemingly endless nerding out over nuance in store for the aspiring vermouth aficionado. But for now let's start at the beginning, with the stuff we're going to call sweet vermouth.