The ahkam al-khamsa, the five rulings, are the categories into which Islamic jurisprudence sorts every human act. Fard or wajib is obligatory: rewarded when done, punishable when omitted. Mustahabb, also called mandub or sunna, is recommended: rewarded when done, not punished when left. Mubah is permitted and indifferent, carrying neither reward nor blame. Makruh is disliked: better avoided, but not punished. Haram is forbidden: punishable when done, and rewarded when abstained from for God's sake.
The scheme's most consequential feature is the size of its middle. Most of ordinary life falls under mubah, and the jurists' working presumption in transactions and customs is permission unless a text establishes otherwise, which is why Islamic law expanded readily into commercial forms that no scripture mentions.
The Hanafi school alone divides the obligatory in two. Fard rests on certain evidence, the Qur'an or mass-transmitted report, and its denial is unbelief; wajib rests on probable evidence such as a single-chain report, and its neglect is sin but its denial is not unbelief. The standard example is the witr prayer, obligatory for Hanafis in the second sense and strongly recommended for the other schools. Hanafis likewise split the disliked into makruh tahriman, close to the forbidden, and makruh tanzihan, merely unbecoming. The other three Sunni schools treat obligatory as one category, and the difference is a good illustration of how a school's theory of evidence reappears as a difference in its vocabulary.