Introduction
The three terms get used interchangeably which can be unhelpful. The distinctions take two minutes to learn. Understanding the differences will help you to become a more intentional reader.
Scanning is searching
Scanning means moving through text to locate a known target such as a name, a date, a figure, a section number. The text in between is not read. Comprehension of it is low, because comprehension is not the purpose. Scanning is Ctrl+F performed by eyes.
It is the right tool for finding the clause, a citation, or a statistic inside a document where you do not need to read everything.
Skimming is sampling
Skimming means passing over a text to extract its gist. You take the headings, the first sentences of paragraphs, the introduction and the conclusion etc and you assemble an impression of what the text says and how it is built. The output is a map of the text. You could give an overview of what the text says now.
It is the right tool for deciding whether something deserves a thorough read and orienting yourself before one. Skimming before reading is one of the highest-value habits in academic work to become an efficient reader. You can skim-read to make a second, thorough read quicker and more effective.
Speed reading is reading
Speed reading means reading quickly. You read every sentence with high levels of comprehension at speed. The mechanics differ from untrained reading, with fewer wasted regressions, reduced subvocalisation and wider chunks per fixation.
It is the right tool for understanding material thoroughly efficiently.