There are three styles of reading which we use in different situations:

• Scanning: for a specific focus
  The technique you use when you're looking up a name in the phone book: you move your eye quickly over the page to find particular words or phrases that are relevant to the task you're doing.
  It's useful to scan parts of texts to see if they're going to be useful to you:
  * the introduction or preface of a book
  * the first or last paragraphs of chapters
  * the concluding chapter of a book.

• Skimming: for getting the gist of something
  The technique you use when you're going through a newspaper or magazine: you read quickly to get the main points, and skip over the detail. It's useful to skim:
  * to preview a passage before you read it in detail
  * to refresh your understand of a passage after you've read it in detail.
  Use skimming when you're trying to decide if a book in the library or bookshop is right for you.

• Detailed reading: for extracting information accurately
  Where you read every word, and work to learn from the text. In this careful reading, you may find it helpful to skim first, to get a general idea, but then go back to read in detail. Use a dictionary to make sure you understand all the words used.

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retrieved from: http://www.studyskills.soton.ac.uk/studytips/reading_skills.htm