Card sorting is a participatory design technique that can be used to explore how people group items into categories and relate concepts to one another whether for digital interface design or a table of contents.
Card Sorting is Day 10 of 100 Days of UX, an exploratory effort to survey the 100 methods of design outlined in Martin and Hannington's Universal Methods of Design. For 100 consecutive days, I learn one new method a day and write about it.