mouseX and mouseY:
Define where the mouse (finger) is in relation to the window or screen.
Follow the cursor around:

        void setup()
        {
            size(500,500);
            fill(255,0,0);
            frameRate(10);
        }
        
        void draw()
        {
            rect(mouseX,mouseY,50,50);        
        }

width and height:
Contain the width and height of the sketch window. They are only set after size() is called (in the standard version, with Android it may be set already).

        void setup()
        {
            size(400,200);
            println("The width is: " + width);
            println("The height is: " + height);
        }

More information about the screen dimensions and other built-in variables available on Android can be found on the Processing Android Wiki Page