Timbre

Timbre - The unique characteristics of a sound produced from any instrument.

A minor change in the timbre can ultimately change how people perceive a tune.  Each different timbre is altered by the overtones associated with it. For example, a flute playing at exactly the same pitch and volume as a piano can easily be distinguished due to its unique sound.

Human voices all have unique timbres, despite some being similar. We can listen to someone speaking and recognise instantly who they are because of the timbre of their voice.

Guitar tone
The sound produced by an electric guitar is influenced by a number of things:

Strings - Thicker guitar strings tend to produce higher volume and longer sustain.

Action - The distance between the strings and the fretboard alters the sound produced. A higher action usually produces a louder volume and a fuller tone, despite being more difficult to press down.

Plectrum/finger picking - Plectrums tend to sound more powerful than fingers when plucking strings; finger picking produces a softer tone. Also, the area in which the string is plucked can also make a difference. If the guitarist plucks the middle of the length of an open string, a mellow tone is produced. The further away from the middle, the harsher the tone produced. 

Pickups - The type of pickup installed on an electric guitar influences the tone greatly. Single coil pickups tend to have a brighter, sharper tone whereas humbuckers are known for sounding warmer and louder.

Amplifier - Different guitar amplifiers affect the tone in different ways. For example, some amplifiers produce a more distorted tone than others. 

Effects - Sound effects are often applied to electric guitar to produce unique or recognisable sounds. Tom Morello, for instance, integrates various sound effects in studio recordings and live performances to produce sounds that aren't noticeably electric guitar. Similar to a synthesiser, guitars can be used as tools to produce a variety of uncommon sounds when combined with sound effects.

Room - The area in which any sound is heard affects it greatly. For example, the acoustics in a studio are designed to muffle any excess reverberation, whereas the acoustics in an opera house are designed to carry sounds throughout the whole room. Artificial reverb is built in to some guitar equipment so guitarists can recreate the atmosphere of a much larger room if they need. 

The tone produced by some musicians compensates for their lack of technical ability. For example, David Gilmour might not be considered technically advanced compared to other guitarists but his appropriate choices of guitar tones within different contexts helps to create popular music. This often appeals to audiences more than the technical ability.