Sintaxis definitions

1. new Sprite();

// by itself it means little... it would have to be assigned as an object as in: var cont:Sprite = new Sprite();

2. stage.addChild(cont);

// ths line is taking an object instance named "cont" and is adding it to the display list (making it visible)

3. cont.addChild(newMedia);

// this line is doing the same thing as the last line except rather than adding it to the stage, it is adding it to the "cont" object

Also, based on what your code shows, cont is not really a variable as much as it is a reference to an instance of some object.  When you create an object using code you use var to declare it.