You can read about him here. In brief, he developed the system of using prophetic hadith interpreted by analysis and human reason to establish a system of Islamic law. Today the other schools of Sunni Islamic law all work within this framework to a greater or lesser degree. The mosque and mausoleum is part of a vast area of mausoleums south of the Citadel, and attracts many poor people seeking alms, as well as many who sit within the complex reading Qur'an and hadith. As with most Muslim religious figures, the actual grave is within a green glass and metal structure, but there was obviously some way to deposit money and written notes within.