Sunnah is to adopt and follow the ways of Prophet Muhammad, from personal conduct to social conduct.. these are reported in researched books that were written on life of prophet muhammad. there is a mechanism(chain of reporting, repetition etc ) that was adopted to determine the authenticity of the precepts or sayings.
Islamic Shariah is the Islamic jurisprudence driven from "certain" Quranic verses, certain precepts of Prophet Muhammad, Qiyas (logic), and Ijma(consensus).
Qiyas is used when one doesnot find the answer in Quran and sayings of Prophet Muhammad. its intelligent guess or deduction of principle or use of logic and reason.
Ijma is consensus on an issue of the jurists.