You can't buy scan milk. :D OK seriously now; I guess this relates to an upcoming English exam. When you skim a text, you are looking for keywords and context to give yourself a general idea of what the text is about: a gist. When you scan a text, you are looking for specific information. In both cases you don't read the whole text thoroughly, of course.

To skim a text means to find out its main idea or the theme of that text. When you scan through a text so you look for specific information .

To skim is to take something from the top, like cream from milk. To scan is to look or to copy, scan the area for intruders, scan these documents then print a copy for everyone. Scan THROUGH these documents and see if the look ok.

speaking about written material, when you scan you look through quickly it in order to find important information. when you skim you just read through it quickly.