Tuning a piano can be accomplished aurally ("by ear") or with the assistance of a computer or other electronic tuning device (ETD). My background and training is in aural tuning, so I tune exclusively by ear.
Aural tuning begins with establishing a careful balance of musical intervals in a process known as "setting the temperament." This starts with choosing a reference note (typically the A above middle C) and tuning it to an external pitch source (such as a tuning fork). The modern pitch standard for the A above middle C is 440Hz, and this pitch is usually referred to as "A440." Once set, the temperament is then spread throughout the rest of the piano. Finally, groups of strings corresponding to the same pitch are tuned to sound clean and pure when played together. The result of this careful labor is a musical scale that suits an individual piano's exact characteristics and design, i.e. a custom tuning. A typical tuning appointment takes around an hour and a half to two hours.
Recommended frequency of tuning depends on a variety of factors including the quality, age, and condition of the piano and the frequency and intensity with which it is played. However, the primary reason pianos go out of tune is due to changes in relative humidity (see the section on Humidity Control for more information). For this reason, pianos should be tuned at least annually to correct for the drifts in tuning caused by the changing seasons. Heavily used pianos such as those in schools and churches should be tuned a minimum of twice a year.
For more information about piano tuning, click here.
Portions of the above service descriptions are adapted from technical bulletins published by The Piano Technicians Guild, Inc.