Requirements engineering is crucial to software development but lacks a precise definition of its fundamental concepts. Even the basic definitions in the literature and in industry standards are often vague and verbose. To remedy this situation and provide a solid basis for discussions of requirements, this work provides precise definitions of the fundamental requirements concepts and two systematic classifications: a taxonomy of requirement elements (such as components, goals, constraints...) ; and a taxonomy of possible relations between these elements (such as "extends", "excepts", "belongs"...). The discussion evaluates the taxonomies on published requirements documents; readers can test the concepts in two online quizzes. The intended result of this work is to spur new advances in the study and practice of software requirements by clarifying the fundamental concepts.