"A vignette is a long-form guide to your package. Function documentation is great if you know the name of the function you need, but it’s useless otherwise. A vignette is like a book chapter or an academic paper: it can describe the problem that your package is designed to solve, and then show the reader how to solve it. A vignette should divide functions into useful categories, and demonstrate how to coordinate multiple functions to solve problems."
Lots of places to look for examples of vignettes.