What is the difference between Usul al-Din (fundamentals of religion) and Usul al-Fiqh (fundamentals of Islamic jurisprudence)?

The meaning of the terms "Usul al-Din" (fundamentals of religion) and "Usul al-Fiqh" (fundamentals of Islamic jurisprudence) differs. Linguistically, "asl" (singular of usul) means that upon which something else is built.

Usul al-Din: This refers to the science of Tawhid (monotheism) and Aqidah (creed), as religion is built upon them. It may also refer to the pillars of Islam and faith, or to the topics of creed, or to those rulings of Islam concerning which a Muslim is not excused for being ignorant.

Usul al-Fiqh: This is a science through which one learns how to derive subsidiary شرعي (legal) rulings from their detailed proofs.