Skimming is used to quickly identify the main ideas of a text. When you read the newspaper, you're probably not reading it word-by-word, instead you're scanning the text. Skimming is done at a speed three to four times faster than normal reading. People often skim when they have lots of material to read in a limited amount of time. Use skimming when you want to see if an article may be of interest in your research.
Scanning is a technique you often use when looking up a word in the telephone book or dictionary. You search for key words or ideas. In most cases, you know what you're looking for, so you're concentrating on finding a particular answer. Scanning involves moving your eyes quickly down the page seeking specific words and phrases. Scanning is also used when you first find a resource to determine whether it will answer your questions. Once you've scanned the document, you might go back and skim it.