What is the main difference between phonetics and phonology? ?What is the definition of and diachronic?

PHONETICS= The system of sounds of a particular language.

PHONOLOGY= The study of the distribution and patterning of speech sounds in a language and of the tacit rules governingpronunciation.

A DIACHRONIC APPROACH is one that analyzes the evolution of something over time, allowing one to assess how that something changes throughout history.

However, there is also A SYNCHRONIC APPROACH which analyzes a particular something at a given, fixed point in time.