Tuning a guitar is simply turning the tuning peg to tighten or to loosen the strings to achieve E, A, D, G, B, and E tunes by using your sense of hearing. However, there are guitar tuners available in music stores to help you indicate the EADGBE. It will sense all keys, the sharps (higher) and flats (lower) also.
In the other hand, there is a tuner we call, chromatic tuner wherein it has twelve notes of scale that will show what the note you are striking in the display screen. They are more complex than the usual guitar tuners because it offers pitch calibration, tap tempo metronome, frequency range for guitars and basses just like snark guitar tuner at guitar center. It has full color display that rotates 360 degrees for easy viewing.
Tuners often have input jack to detect its pitch that sometimes any background noise from other harmony coming to musical instrument can complicate the tuner. Most of the input jacks are for electrical instruments. When it comes for guitars, there is a clip-on tuner to bring convenience for the musician to sense frequency. Snark also has there clip-on tuners and it is also colored-display.
Those are just the basic facts about instrument tuning so strum and tune!