Whether you play rock, jazz, classical music or any other style, it's always important to be in tune. You may be able to fly around the guitar, bass or sax, and you may have mastered many impressive techniques, but if your instrument is not well tuned, the result will still sound painful!
Electronic tuners are small, normally hand-held devices which generally fall into one of two groups: instrument-specific designs (e.g. guitar/bass), and universal chromatic tuners for all instruments. There are also a few more highly specialised models, for example historical performance tuners with greater control over aspects such as temperament.
All tuners measure the pitch of an incoming signal by analyzing the waveform and display this information in one or more ways.