What is the difference between 'de-noising' and what we commonly know as filtering methods to remove noise?
De-noising is about the goal, and filtering is about the technique you employ. 

You can obviously de-noise via filtering. For example, if you know that your system cannot transmit frequencies above a certain threshold, you can apply a low-pass filter. However, you can de-noise by other techniques as well, such as by averaging multiple recordings of a signal. Similarly, you can apply filters with other goals than (just) noise reduction. 

Note that "noise" may not necessarily be additive random noise - depending on your context, non-pertinent signals may be lumped with noise as well, such as the sound of vuvuzelas.