The use of non-harmonic tones is probably the most expressive technique at the composer's disposal to flag up tension within and between triads in a progression. Although usually mentioned by the composer, they can also be added in performance as an ornamental device to spice up a melody. Non-harmonic tones can be either diatonic or chromatic and can be used at the same time with one another to provide even more tension. All non-harmonic tones are prepared and resolved by consonant tones. I have placed them into categories depending on their metrical/rhythmic articulation and the resultant emphasis.
So before we get in the grip with those non-harmonic tones, let's mention some basic knowledge.
Essential notes are notes that belong to the chord. Chromatic nonessential notes are notes that belong to the key.