The trombone is a brass horn musical instrument. It is similar to a large trumpet, except the player pushes and pulls on its slide to change the length of the tube.

Together with vibrations from the player's lips, the trombone can play a wide range of notes. It sounds deeper than a trumpet and is usually said to be one of the bass clef instruments.  Music is normally written in the bass clef, but some high parts can be written in tenor clef. Some players have the music transposed into the treble clef. The trombone is the loudest instrument in the symphony orchestra. 

A person who plays the trombone is called a trombonist. Some trombones have a valve which increases the range of available notes. The name trombone comes from the Italian language and means large trumpet. Trombones are nearly always made out of brass but can also be made out of plastic: the "P-bone".

History 
The forerunner to today's trombone was called the sackbut.  It was first used in the 16th century during the Renaissance era of music.  In the centuries that followed, the sackbut was gradually improved into today's trombone. Beethoven was the first composer to add trombones to the standard symphony orchestra. Many different types of music use the unique sound of the trombone to add color and depth.  Trombones are used in orchestras, concert bands, marching bands, brass bands, big bands, swing bands, ska bands, and jazz ensembles. In the symphony orchestra, it is common to see trombones in a section of three: two tenors and a bass. In a standard big band, a four player section is more typical. This time with three tenors and one bass as the normal arrangement.

In the hands of a skilled trombonist, the trombone is a very versatile instrument and can play many styles. The trombone can go from smooth, sweet jazz ballads and peaceful slow melodies such as Duke Ellington's "Pyramid", to fast and technical passages from pieces like "Blue Bells of Scotland" and "Carnival of Venice".

Types 

There are a lot of types of trombones. Each one is for different playing ranges (how high or low you play). There are contrabass trombones, bass trombones, tenor bass trombones, tenor trombones, alto trombones, soprano trombones, and piccolo trombones. These are the different types of trombones, arranged according to general usage.

The tenor trombone is the most popular trombone type in use today, and is usually what people mean when they say "trombone". It is usually the first instrument a new trombonist will play. It has a slide that is used to change notes and make music. People who play the tenor trombone usually play from bass clef but some very good players can play from treble clef. Better and more expensive tenor trombones have an F attachment.
The bass trombone is different from the tenor trombone because it has one or two triggers, and has a larger bell, which makes it sound better in lower octaves. Most bands have only one bass trombonist. Usually, if a song does not have a bass trombone part, the bass trombonist reads off the 3rd or 4th trombone part.
The alto trombone is a type of trombone that is somewhat higher sounding than a regular trombone. It is mostly used in orchestras where the highest part is sometimes easier to play on an alto trombone. However, it is not very common and most of the time, a tenor trombone  plays the alto trombone part if one exists.
The contrabass trombone is the lowest-sounding trombone used in music today. It sometimes has a double slide, meaning that the slide tube wraps twice instead of once like on a tenor trombone.
The soprano trombone is a type of trombone that plays in the same range as a Bb trumpet. 
The valve trombone is just a tenor trombone with a valve section. These valves work the same as those on a trumpet, euphonium, etc.

Famous trombonists
Some famous trombonists include:
 Edward ‘Kid’ Ory 
 Bill Watrous 
 Carl Fontana 
 Jim Robinson
 Jack Teagarden
 Al Grey
 Glenn Miller
 Tommy Dorsey
 Pete Ramberg
 Lawrence Brown
 Curtis Fuller
 Slide Hampton
 Fred Wesley
 Trombone Shorty
Martin Schippers
 Christian Lindberg
Joseph Alessi
Christopher Bill
Wycliffe Gordon

References

Other websites 

 International Trombone Association
 Trombone Page of the World
 TrombonesOnline   - Trombone Portal and Trombonist Directory
 British Trombone Society
 Finnish Trombone and Tuba Association 
 Online Trombone Journal