Mutes are widely used additions to instruments, usually for jazz, but which can also be seen in chamber and orchestra music. There are many different types, each eliciting a different sound, nuance, or effect, but each are used in a similar manner. All that I know of are for brass instruments, and almost always for use in trombones or trumpets. Each is inserted into the bell, in order to stop or alter the sound exiting the end of the instrument.

Some common mute types (all vary by size according to instrument):

Plunger: Perhaps the most common mute, this is simply the rubber end taken off any (un-used) toilet plunger. There are more expensive, made-for-music models, but they pretty much do the same thing. Held at an angle over the bell, as to not totally block off all sound, the plunger will give a softer, reverberating nuance to the tone of the note. Can also be used to create a "wah-wah" effect by moving the hand and mute closer or farther away from the bell. In the event that one forgets his or her plunger, the player's hand may be substituted, but only in emergencies.

Straight Mute: Another very common mute, this comes in a conical shape which will be partially stuck into the bell. It is usually fashioned out of high-grade cardboard and stone lining. Peices of cork and pressure hold it in place. The straight mute gives the tone a softer and slightly nasal sound.

Cup Mute: Very similar to the straight mute, the cup mute has all the same attributes except for a cup at the end which comes up to about 1/4 of the way up the cone. This creates a much more nasal sound, and also creates a more "tinny" effect on the note played.

Harmon Mute: The Harmon mute is usually made out of fiberglass or aluminum. Most used by trumpet players, and most often in jazz. This mute creates a more "tinny" sound (especially if aluminum), and also creates a buzz due to a stopper in the center of the front of the mute. This can be removed to create less of a buzz, or moved in and out to create a "buzz-wah" effect.

Bucket Mute: Also held in by a cork pieces on a tube-like arm into the bell, this mute has a bucket off the end that traps much of the sound. This creates a warm or fuzzy nuance in the note, along with the quieting effect. Other styles are simple cylindrical buckets which clip to the outside of the bell (this is the more classic style).

There are, of course, many more variations involving shapes, sizes, and especially materials. Another, more boring mute is the practice mute which stops all sound (obviously for the purpose of practicing, in order to avoid annoying everyone in your general vicinity).