Non-destructive editing technique used in photoshop is called Masking. Here certain layers of an image are masked or hidden revealing only what is required. But the hidden part is always available and never completely deleted. photoshop offers five different masking methods – Layer Mask, Quick Mask, Vector Mask, Channel mask and Clipping mask.
To edit an image in photoshop, the lasso tool is commonly used. This provides very rough editing. The eraser tool can be pretty time-consuming cumbersome and irreversible as well. Masking makes photo and image editing simple and convenient without loss of any original data.
                    

In a layer mask, portions of a layer can masked. Anything that is black in the mask is hidden in the layer and anything that is white is visible.
In a quick mask, the masked areas appear in red and selection can be made accordingly.
In a vector mask, vector shapes are used to create the masks. Pen tools and paths are used to create the vector shapes and these are more accurate then the pixel masks.

 
Clipping Mask
In a clipping mask, the required portion in a top layer can be clipped onto a bottom layer, thereby choosing the required portion to be clipped for visibility masking out the rest of the content.
All in all the masking techniques offered by photoshop make it a very efficient photo and image editing software.

Ive sent this user a first warning and this PM

"Dear adarshan1
 I'm sad to inform you I had to remove your recent post "MASKING IN photoshop"
 for massive advertising of a (very expensive) commercial product. This is not allowed on Posetteforever and will not be tolerated. If you'd like to share your experiences and techniques in picture editing on an product independent level or using free, open source software, you are welcome of course.
 Your's
 ahjah"

Also I had to edit his signature containing a link to a commercial animation school

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This user got the second warning for re-installing the link in his signature

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