An app (application or also sometimes called widget) is a program you can add to a web page, blog, wiki, computer desktop, Facebook, or MySpace – to name a few. In the space the user selects, she can incorporate apps in order to have a calendar, booklist, to do list, library catalog search, etc. easily accessible in that space.
Adding an app is usually pretty simple and can be accomplished in one of two ways. One is to click a button, tell the site where to add the widget code, and then the apps is included on the page. The other is to copy the code that makes the widget work and paste it into a web page or social network.