The principal brass instruments of the modern symphony orchestra are the FRENCH HORN (also called HORN), TRUMPET, TROMBONE, and TUBA.
Brass instruments are wind instruments, and although they may be coiled or bent into different shapes, all brass instruments consist essentially of a very long metal tube. If you straightened out all the tubing on a French horn, for example, it would be about seventeen feet long. The trumpet would be four to five and a half feet, the trombone nine to thirteen feet, and the tuba sixteen to twenty-six feet. A mouthpiece is inserted in one end of the tube; the other end of the tube flares outward and is called the “bell.” All modern brass instruments use a cup-shaped mouthpiece, the specific configuration of the cup varying with each instrument. The brass player doesn't simply blow into the mouthpiece, however, he buzzes, compressing his lips and forcing air through them so that they vibrate together. The vibrations of the lips, focused and amplified by the mouthpiece and directed into the tube, cause the column of air inside the tube to vibrate, and as in all wind instruments, the vibrating air column produces the sound of the instrument.
The FRENCH HORN is called “French” only in English. Nobody is entirely sure why it's called French, especially since the French themselves simply call it cor (“horn”); the Italians, corno; and the Germans, Horn. A likely theory is that the English originally called the instrument the French horn (in the 1600s) for the simple reason that it came from France. And even in English, the French horn is often just called the “horn.”
The TRUMPET is the highest-pitched and most brilliant member of the brass family. Like early horns, early trumpets were “natural” instruments, without valves. The earliest trumpets, used as signaling or ceremonial instruments in many ancient cultures, were very natural: they were made of such things as ram's horns, ox horns, seashells, and elephant tusks. Metal trumpets (silver, bronze, brass) of the Middle Ages were generally long straight tubes with flared bells; it wasn't until the fifteenth century that instrument makers learned to bend brass tubes, which allowed them to make the instruments more compact. The first bent trumpets were in the shape of a flattened S-curve, but they were soon made obsolete by trumpets whose tubing was fashioned into an elongated oval coil. Instrument makers of the time also invented a trumpet with a sliding, telescoping tube extension. This “slide trumpet” survived in specialized roles for several centuries, but it's mainly remembered now as the precursor of the trombone. With various modifications, the oval coil has remained the basic shape of the trumpet to this day.
The TROMBONE (also sometimes called SLIDE TROMBONE ) has remained largely unchanged for over five centuries. Trombone is Italian for “big trumpet”: instrument makers in the fifteenth century created the trombone as a bigger, lower-pitched, and improved version of the slide trumpet. Up until the eighteenth century, the lovely English name for trombone was sackbut, a word of Old French or possibly Spanish derivation.
The TUBA is the biggest, heaviest, widest, and lowest-pitched of the brass instruments, the true bass of the family. It's also by far the youngest member. It was invented in Germany in the 1830s and has never been a “natural” instrument — it has always had valves. Like the trombone, the tuba comes in several different sizes. The size most commonly used in the orchestra is called the contrabass-tuba, or double-bass tuba. Solo opportunities for tuba players outside the orchestra are few, although the English composer Ralph (pronounced “Rafe”) Vaughan Williams wrote a fine concerto for tuba and orchestra (1954). It's not surprising, in view of his many innovations in orchestral instrumentation and sonority, that Hector Berlioz (1803–1869) was the first important composer to write for the tuba.
Taken from Chicago Symphony Orchestra's website.