There are many ways to classify ahadith. One method is by authenticity: is it saheeh (authentic beyond question), hassan (good), da'eef (weak), or mawdoo' (fabricated?)
In my answer on another question, I mentioned a hadith that's recorded in At-Tirmidhi, which is Hassan (according to my source). Although that example did not have a hadith which scholars derived a ruling from, it raised this question:
What kinds of ahadith do scholars derive fiqh from, in terms of authenticity?
I'm not interested in that discussion about tawaatur, so please don't mention that in your answer.