Shiny was originally conceived as a medium for interactively communicating ideas and results. As the author of a Shiny app, you perform your analysis or build your models, and then write a Shiny app to let other people–especially those without R expertise–explore your findings or data.
Shiny Gadgets are different. Gadgets are interactive tools that enhance your R programming experience.
Where a Shiny app represents the output of an analysis, Shiny Gadgets are designed to be used in the course of analysis.
Where Shiny apps are designed to be used by end users, Shiny Gadgets are intended to be used by R users.
Where Shiny apps are ultimately intended to be deployed on servers (like Shiny Server or ShinyApps.io) and accessed via a web browser, Shiny Gadgets are only ever intended to be invoked from code (in the R console or from an R script) or from