What is the exact difference between read, scan, and skim? Is the difference related to the concentration on the text, or is there anything else?
The answer is really important for me, since I often see these words in the texts I read.
For what it's worth this was written "tongue-in-cheek."
Incidentally, the terminology does fit.
OP doesn't give any context, so - lighthearted or not - @Mr.Wizard's answer at least sets out how these three words have fairly distinct and well-understood meanings in the context of various "security" cards (credit cards, library cards, etc.).
In the context of books and other "volume" text, the verb to read is a generic term, which can mean anything from a brief glance to exhaustive textual analysis.
For most practical purposes, skimming and scanning are synonymous in reference to a book, for example. They just mean "reading" the text rapidly and superficially - often skipping words, sentences, paragraphs, pages, or even whole chapters to gain a quick overview.
Note that there's also the possibility of scanning a book into electronic format using electronic hardware, perhaps OCR (optical character reader/recognition) software to convert the scanned image into searchable text.
Thanks to @onomatomaniak for pointing out below that in some contexts, scan could be used to mean rapidly "sight search" a body of text looking for some particular term of interest. Such an activity has little connection to "reading" as such - you might well absorb no information at all, apart from establishing whether your "search term" is actually present.
In the context of a book, skimming is a technique used to quickly absorb the highest level of abstraction of the content of the book, for instance by browsing the content, chapter titles, summaries, etc. Little or none of the detail of the book will be gained during skimming.
Scanning is to search for a particular word, phrase, diagram or other piece of information of interest within a book, possibly by using the index or table of contents (or alternatively, using a search for keywords if the content is electronic).
One would use reading in order to absorb the total content and meaning of the book, usually by reading the content from start to finish, in a linear fashion, without skipping anything.
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