Skimming and scanning are two very different strategies for speed reading. They are each used for different purposes, and they are not meant to be used all the time. They are at the fast end of the speed reading range, while studying is at the slow end. What Is Skimming? Skimming is one of the tools you can use to read more in less time. Skimming refers to looking only for the general or main ideas, and works best with non-fiction (or factual) material. With skimming, your overall understanding is reduced because you don’t read everything. You read only what is important to your purpose. Skimming takes place while reading and allows you to look for details in addition to the main ideas.Skimming will help you locate the information quickly while making sure you use your time wisely. It will also increase the amount of usable material you obtain for your research. Suppose you have an exam in a few days. You need to review the material you learned, but you don’t want to reread everything. By skimming, you can quickly locate the information you haven’t mastered yet and study only that material.What Is Scanning? Scanning is another useful tool for speeding up your reading. Unlike skimming, when scanning, you look only for a specific fact or piece of information without reading everything. You scan when you look for your favorite show listed in the cable guide, for your friend’s phone number in a telephone book, and for the sports scores in the newspaper. For scanning to be successful, you need to understand how your material is structured as well as comprehend what you read so you can locate the specific information you need. Scanning also allows you to find details and other information in a hurry.

They are both techniques used to read documents quickly. This BBC page explains them quite well with examples http://www.bbc.co.uk/skillswise/factsheet/en05skim-e3-f-skimming-and-scanning

When you skim a text, you just read it quickly, for the general ideas (for "the gist"). When you scan a text, you are looking for specific information. In both cases, you don't read the text fully. You skip through it.

As an aside... when studying from a textbook, rather than reading a chapter straight through, first I flip through the pages and read all of the big subsection headings. Then I do it again, and this time I read all of the smaller section headings. If there are even smaller headings, I go through and read those. Then I do it AGAIN, and this time I read the first sentence of every paragraph. Then finally I settle down and read from beginning to end.