ok. So the question again, what is photoshop?
Well, I already clarified that it is a raster graphic editor, so firstly, the answer to 
"what is a "raster graphics editor".
Well it is simply a program that enables the creation or editing of images on screen, in a pixel based format. Think of the 'paint' program that you may well of encountered, installed on your windows PC. Think of it as an electronic canvas.
Photoshop is an advanced version of paint, with many more features, but the primary important one being that it uses 'layers'. 
You can use these layers to continue to stack  them  on top of each other - or place them underneath. Layers are pretty much like pieces of tracing paper.
They are by standard completely transparant.
Photoshop, and indeed all editing programs work with bitmapped graphics.
The drawback to bitmapped graphics is they cannot be re-sized as effectively as
a drawn object created in a drawing program known as vector graphics.
Vector graphics can be re-sized to virtually any size without any resolution breakdown. Or put simply, vector graphics can be blown up to any size without a loss of visual quality.
All screens display in the bitmap graphic format, even if they are vector graphics.
Photoshop was created in 1988, and quickly established itself as the industry standard. It has been such a game changer in the photo editing industry, that most people have probably heard the term that an altered  image has been 'photoshopped'.
When an image/photo is imported into photoshop, you are effectively loading a digital image made up of thousands of pixels, onto your digital canvas (your screen). You then 'paint' over the pixels you want to edit/change/remove;using another layer- or your tracing paper, and continue this as many times as needed.
The tools at the users disposal are many and some are very complex. 
With an experienced editor and photoshop, there is little that cannot be achieved.

August  25, 2018

So what is Photoshop?....
Well, the proper 'technical' answer is, "a raster graphics editor ",
but what is a raster graphics editor?
In the following artical I shall attempt to explain, where possible,
in layman's terms exactly that.