What is the difference between Fiqh and Sharia? And what is Usul al-Fiqh?

Shari'ah, linguistically: A permanent, evident watering place. Technically: The entire religion that Allah has chosen for His servants, which He has legislated and clarified for them in terms of commands, prohibitions, and lawful and unlawful matters.

Fiqh, linguistically: Understanding. Technically: Knowledge of the practical, subsidiary, Shari'ah rulings derived from detailed evidences.

Usul al-Fiqh, linguistically: The origin (asl) is that upon which the existence of something relies. Technically: The science of the evidences of Shari'ah rulings and their general modes of indication, or a science that investigates the general evidences of Fiqh, how to benefit from them, and the state of the beneficiary.