A tuner is a device that detects and displays the pitch of musical notes played on a musical instrument. "Pitch" is the highness or lowness of a musical note, which is typically measured in Hertz. Simple tuners indicate—typically with an analog needle-dial, LEDs, or an LCD screen—whether a pitch is lower, higher, or equal to the desired pitch. In the 2010s, software applications can turn a smartphone, tablet, or personal computer into a tuner. More complex and expensive tuners indicate pitch more precisely. Tuners vary in size from units that fit in a pocket to 19" rack-mount units. Instrument technicians and piano tuners typically use more expensive, accurate tuners.[1]
source: Wikipedia