In order to sound good while making music together, voices and instruments must match their pitches precisely with the other voices and instruments in the ensemble. The process of matching pitches very precisely is called tuning.
For voices, all tuning is done with the body. Instruments, on the other hand, must be carefully manufactured to play in tune with particular tuning systems. Most instruments also require additional tuning after they are built. Keyboard instruments like the piano are tuned by professional tuners using precise equipment. Players of woodwind, brass, and string instruments tune their instruments prior to each performance by adjusting the tension of strings, the length of tubes, or the position of mouthpieces. During performances these instrumentalists (and vocalists, too!) must continue to hone and maintain correct tuning, sometimes with small changes in technique and sometimes by making further adjustments to their instruments.
A Reference Point: A 440
A4 = 440 Hz
Before an ensemble performs the musicians must tune their instruments to a common pitch. In modern tuning systems, the pitch A4 (the A above middle C) is defined as 440 Hz and is the reference point for all other pitches. Because it is so central to tuning, this is the pitch usually used for pre-performance tuning. A 440 is also a useful reference point when discussing tuning systems.
Audio examples are triangle waveforms downloaded from OnlineToneGenerator.com.