PhantomJS is practically a headless web browser that contains a javascript API, but it is headless, which means that has no window nor viewport, its practically a command line where you feed javascript-bases programs in order to test your functionality.
PhantomJS is practically a headless web browser. This headless web browser contains a javascript API. It is headless. Headless means that has no window nor viewport where you feed javascript-bases programs in order to test your functionality. Viewport is its practically a command line.