Confusion over different kinds of secondary research, and their divergent purposes, is undermining the effectiveness and usefulness of secondary studies in software engineering. This short paper therefore explains the differences between ad hoc review, case survey, critical review, meta-analysis (aka systematic literature review), meta-synthesis (aka thematic analysis), rapid review and scoping review (aka systematic mapping study). These definitions and associated guidelines help researchers better select and describe their literature reviews, while helping reviewers select more appropriate evaluation criteria.