Answer:
Skimming: Skimming will help you grasp the general idea or gist of a text. You might quickly read the table of contents, the headings or the abstract. You could also read the first and last paragraphs and the first and last sentence of each paragraph in a relevant section.
Scanning: Scanning allows you to locate precise information. You might identify a key terms or expressions which will alert you to where your subject is being addressed. You could then run your eyes over a text looking for these.