What with the invention of valves being only a fairly recent (in historical terms) development for brass, the slide trombone is one of the oldest and most unchanged instruments that are used today. Not in fact starting with the Sackbut (there are models similar to the modern trombone from before the times of the sackbut), the trombone has been around for at least 500 years, perhaps more. Obviously, it is near impossible to find a sackbut that was created in the Tudor days but the trombone has been redesigned a little since then.
Although the general design of the trombone has not changed a lot, there have been several small changes in certain elements of the trombone. For example, in Beethoven's day, trombones had a smaller bell; this meant that the sound was closer to that of a tenor (or Eb) horn than a trombone, especially with louder notes. Bass trombones, also, have undergone a fairly important change in their design; in older times, bass trombones were equipped with a stick, with which the player could extend the slide beyond the reach of his arm, making for lower notes. Recently, a trigger was developed which sent the air round more tubing (as is attached into the space at the opposite end from the slide), effectively changing the key of the trombone from Bb to F. This means lower notes can be reached. Alto trombones, which basically look like baby trombones and are in Eb, have become almost extinct nowadays, whereas Beethoven used them extensively. The trombone has increasingly become an instrument of practicality rather than of ceremony, which is either good or bad, dependant on your view. The modern trombone (without a Bb/F key) has just over a 4 octave range, if the player is up to playing the highest and lowest notes. With a Bb/F trombone, players can access a 4.5 octave range just as easily as a 4 octave, and as a result very good players can get up to 5 octaves with this accessory.
The Slide
The slide itself is, though many non-trombonists become confused with it (a common question is "how do you know where the positions are?"), actually quite a simple concept; as the slide is lengthened, there is more space for air through the trombone, and so the pitch is lowered. Obviously there are implications regarding the player's use of the mouthpiece, which I will come on to later, but as a basic concept moving your arm out lowers the sound.
There are 7 positions, learnt by conditioned reflex, and they go in this order;
The Mouthpiece
Similar to all brass instruments, higher notes are achieved by tightening your lips against the bell, which increases the speed of the air-flow. This is how different pitches are acheived from a single position. Trombone mouthpieces tend to be much larger than that of a trumpet, but smaller than that of a tuba; in fact, the trombone has an identically-sized mouthpiece to that of a euphonium; you CAN get different-sized mouthpieces but they will continue to work on a euphonium, in general.
The player "tongues" different notes to stop the notes running together; for softer pieces, the tongue mimicks the shape for "doo", whereas for harsher sounds the shape for "tah" is employed.
Mutes
As with almost all brass instruments, the mutes for the trombone either cover or go inside the bell. For example, the cup mute, which has a conical (hollow) tube with a cup-like structure at the base fits with the cone inside the bell and the cup covering the bell, with a small gap. This produces a much thinner sound. A plunger mute is used to produce a "doo-wah" sound; the plunger cuts off all air being released until the trombonist takes it away.
Music
There is a wide variety of music for the trombone; it has a rich sound, making it ideal for orchestras, but the curious sound of glissandos on the trombone (moving the slide without tonguing the notes in-between) makes it useable, and indeed important, for Jazz music. More recently, Ska music has also grabbed the trombone's unique qualities.
Trombonists have to, when they acheive a certain level, read all 4 of the modern clefs; Treble, Alto, Tenor, and Bass. This makes reading music for the trombone actually harder than for any instrument; no matter how long you play the trombone for, it is impossible to become fully adapted to all 4 clefs. Trombonists must scan a piece carefully before playing. Treble clef for trombone continues to be slightly more confusing; it is a Bb key signature, and therefore a written C is played at a Bb. Because of this, key signatures are not what they appear to be; Bb major is pictured as without flats (or sharps). Which is fairly confusing. However, it is this clef which is employed in Brass Band music.
Players
Obviously, there are many trombonists in the world. But there are a few famous trombonists around;