To "scan" means to scrutinize closely looking for something SPECIFIC.
A sailor might scan the ocean with a spyglass or binoculars looking for land.
Another example is "scanning" an invoice to look for the final price. We can also scan the telephone book looking for a specific name. If we can say, "there it is", we are scanning.
To "skim" is get a basic overview or GENERAL idea.
Most of us skim over the headlines of the magazine covers on display when we are waiting in line at the grocery store.
When we get a new book, we might skim over the chapters to get the gist of how the story will develop.
Hope this helps