A web app is a piece of software that is embedded in a web site and performs a particular function. It is usually developed as a self-contained unit, and may even use a different programming language than the rest of the site. It allows you to increase the functionality of a website without making it harder to use. As an example, when you post a YouTube video to Facebook you are using at least two web apps to do so. The definition is a little fuzzy, but as a general rule, if it sings, dances or lets you play a game, it’s a web app.