Concept-focused quiz questions required College Algebra students to write about their understanding. The questions can be viewed in three broad categories: a focus on sense-making, a focus on describing a mathematical object such as a graph or an equation, and a focus on understanding vocabulary. Student responses from 10 classes were analyzed. Students' writing provided insight into misunderstandings they had about linear and quadratic equations and systems of linear equations and difficulties using correct mathematical vocabulary. Instructors can use this type of information, which may be undetected with other forms of assessment, to make instructional decisions that meet students' needs.