A "category" is a way to group items or concepts based on shared characteristics or qualities. For example, in a library, books can be categorized by genres such as fiction, non-fiction, or mystery. This helps people find what they are looking for more easily. Categories can also be used in various fields, such as science, where living organisms are classified into categories like mammals, birds, or reptiles. This classification system helps scientists understand relationships and differences among species, making it easier to study and communicate about them.