Organologists, also sometimes called musicologists, are people who study musical instruments. Their studies aren't from the musician's perspective. Rather, their studies are more from a scientific and historical angle. The Virginia Tech Music Dictionary defines it as " The science of musical instruments including their classification and development throughout history and cultures as well as the technical study of how they produce sound."
There was a point in history when music didn't exist. But, someone had to think of these things first.... how to stretch an animal skin over a piece of a hollow log to make a drum; how to hollow out an animal bone to make a horn. Ultimately, organologists were studying instruments long before musicians were making notes for them. No one will ever know for sure, but some of these thought processes may go back four or five thousand years.
Fast forward to about 1800 or so. Blacksmiths are still forging copper and brass instruments by hand. Musicians are changing horn parts (crooks) during song breaks to re-key instruments. By about 1815, somewhere in Europe, the valve is invented and placed on horns... Between about 1815 and 1880, organologist (mainly in Austria, Germany, France, and Italy) research, develop, and prototype more musical instruments than ever before.... or since! Just look around the web at some of the historical collections and you'll get an idea. By the end of the 19th century, the designs of today's modern instruments are pretty well carved in stone.
In 1880, Victor Mahillon, the curator of the Royal Conservatory of Brussels' instrument collection, began to catalog all of the instruments. He published his work in five volumes around 1884 or 1885. Although it was a grand effort, it concentrated on European instruments and had a lot of holes in it. It was, however, one of the first to classify music instruments based on what vibrated in an instrument and it lead to the next round of taxonomy.
In 1914, a couple of organologists, Erich Moritz von Hornbostel (1877-1935) of Austria, and his colleague, Curt Sachs (1881-1959) of Germany wrote "Zeitschrift für Musik." The Hornbostel-Sachs system offers a way to classify any instrument from any place or time period of origin. Hornbostel-Sachs was later translated, revised and published in English in the "Galpin Society Journal" in 1961. Modern day organologists whine about it a lot, but Hornbostel-Sachs it is still the most widely used musical instrument taxonomy system.
Using strict interpretation, Hornbostel-Sachs is based on the Dewey Decimal system. It defines five top level instrument classes, with many (over 300) sub levels. These are the major points (except for the Electrophones):