What is the relationship between Hadith and Sunnah?
Hadith really represents Sunnah. Hadith makes up the Shari’ah. Its interpretation and application is what we called Fiqah (Islamic Law).
When used as a noun, Hadith means Speech and when used as an adjective, it means News. From the Islamic perspective Hadith means sayings, actions and approvals of Prophet Muhammad (ﷺ) which were conveyed by his companions and then the later generations over the period of time, and recorded in the text which we know today as books of hadith.
Definition of Hadith also includes authentic (Sahih) hadith and inauthentic (Daeef) hadith. In relationship to the Sunnah, Hadith represents the vehicle in which the Sunnah was conveyed to us. Sunnah includes things which Prophet Muhammad (ﷺ) did but, they necessarily do not have any legal implication, and it also includes description of Prophet Muhammad(ﷺ) . Usually when we talk about Hadith we do not mean descriptions of the Prophet Muhammad (ﷺ), though these descriptions were conveyed through the Hadith . Sunnah occurred during the time of Prophet Muhammad (ﷺ) some of which were not even conveyed by Prophet Muhammad (ﷺ). These are called Athar . Something which were related to the things done by the companions or circumstances occurred during Prophet Muhammad's (ﷺ) time which were not directly related to the Prophet Muhammad's (ﷺ) actions but conveyed to us by the companions, are called Athaar.
Source: Hadith & Fiqah
 Dr. Bilal Philips , Usool Hadithpg. 2-5
 Usool Hadith pg. 8
Tuhfatul Ahwathiy: Sharh Tirmidhi, Introduction, pg. 6
 Sciences of Hadith, Dr. Mahmood Tahhan, pg.2< Back to Questions