Salam reference wikipedia
Ijtihad (Arabic: اجتهاد, ʼijtihād) is the making of a decision in Islamic law (sharia) by personal effort (jihad), independently of any school (madhhab) of jurisprudence (fiqh). as opposed to taqlid, copying or obeying without question.
To be valid and accepted it has to be rooted in the Qur'an and the hadith and it is required that no established doctrine rules the case. A mujtahid is an Islamic scholar who is competent to interpret sharia by ijtihad. Whereas Akhbari Shi'a Muslim outright reject ijtihad and do not imitate a mujtahid who practice ijtihad
Ulama (Arabic: علماء ʿUlamāʾ , singular عالِم ʿĀlim, "scholar"), also spelt ulema, refers to the educated class of Muslim legal scholars engaged in the several fields of Islamic studies. They are best known as the arbiters of sharia law. While the ulama are well versed in legal fiqh (jurisprudence) being Islamic lawyers, some of them also go on to specialize in other fields, such as hadith or tafseer.