A query is a resource that defines the parameters of the data you want to report on derived from a data source. Queries are used in Reports either directly or as the basis for Trends reporting (How Trends Work).
Queries can use as a data source the database of events, cases, notifications, modeled network objects (assets), trend data, active list, or session list. Reports then bind data results from queries and/or trends into a display format based on a report template.
See Building Reports for an overview of all reporting tasks and tools, including how to build queries or trends and how to use a provided templates.