Skimming - a quick look at the text, in order to have a first impression - an idea. The first and last sentence of each paragraph are particularly important, focus on them. They normally sum up the main point. Scanning - look for hints and/or specific information for comprehension. Spot the keywords, and read the sentences where they appear more frequently. Don't stop until you find the answer you're looking for. Pictures, numbers, symbols, graphics are also important in this case.

Skimming is a reading strategy to find a main idea. To do so, the students are encouraged to read in a fast speed, without paying attention to every details. Scanning is a reading strategy to find specific information. It can be main idea, but most likely searching for specific details.

IMO, no abstract difference, however, implication is all the difference Skim = to speed-read without paying attention to the small details Scan = to speed-read and pick up the small details automatically (there is an implication that if you scanned it, you read it)