Social question and answer (Q&A) Web sites field a remarkable variety of questions: while one user seeks highly technical information, another looks to start a social exchange. Prior work in the field has adopted informal taxonomies of question types as a mechanism for interpreting user behavior and community outcomes. In this work, we contribute a formal taxonomy of question types to deepen our understanding of the nature and intent of questions that are asked online. Our taxonomy is grounded in Aristotelian rhetorical theory, and complemented by contributions of leading twentieth century rhetorical theorists. This taxonomy offers a way to differentiate between similar-sounding questions, while remaining flexible enough to encompass the wide range of questions asked online. To ground the taxonomy in reality, we code questions drawn from three popular social Q&A sites, and report on the distributions of several objective and subjective measures.