Answer:
The words Hadith and Sunnah have been used to mean the same thing. They can also be used to signify different but related things.
A Hadith is considered a saying of Muhammad (peace be upon him). His sayings are part of what makes up the Sunnah.
The Sunnah includes everything. His sayings (plural: Ahaadith), his actions, things that he approved or disapproved of, whether he said it or not. In other words, if something was done in his presence, and the Prophet did not say anything, then it is considered a sign that it is an acceptable thing to do.