This paper describes research on a classification of physics problems in the context of introductory physics courses. This classification, called the Taxonomy of Introductory Physics Problems (TIPP), relates physics problems to the cognitive processes required to solve them. TIPP was created in order to design educational objectives, to develop assessments that can evaluate individual component processes of the physics problem-solving process, and to guide curriculum design in introductory physics courses, specifically within the context of a "thinking-skills" curriculum. Moreover, TIPP enables future physics education researchers to investigate to what extent the cognitive processes presented in various taxonomies of educational objectives are exercised during physics problem solving and what relationship might exist between such processes. We describe the taxonomy, give examples of classifications of physics problems, and discuss the validity and reliability of this tool. (Contains 8 tables and 6 figures.)