Errrmmmm Sharpening in digital image editing is increasing the contrast between different shades or tones the same happens with film by choosing different: film, papers & processing. Unsharp mask the most popular way of sharpening is a traditional photographic technique copied over to digital image processing.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html