<p>I'm trying to print a calibration cube from PLA using a 70&nbsp;&deg;C heated build platform on a Prusa i3 Pro W. This results in:</p>

<p><a href="https://i.stack.imgur.com/G00Um.jpg)" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/G00Um.jpg)" alt="Calibration cube print failed"></a></p>

<p>Please explain why it prints like this.</p>

There are multiple issues that cause this result.
First, your nozzle is to far from the bed. This can be seen by the curly deposited filament on the build plate (I guess that is the brim or the skirt). Please properly level the bed and position the nozzle at a distance of a plain A4 paper as best as possible (should be doable as you have a glass sheet that are usually very flat as a result of the production process to make glass).
The second problem you face is layer shift. You see that the squares are printed further and further to the left, the print head does not return to original position. Layer shift is usually caused by improper belt tension or a loose grub screw of the belt pulley.
This answer
describes layer shifting in more detail.
This question
may be helpful too, the answer contains some references to layer shifting.