The difference between Google and Google Scholar is that Google Scholar focuses on the scholarly literature available on the Internet. This includes, articles, theses, books, abstracts, U.S. court opinions, and patents, found on the websites of academic publishers, professional societies, online repositories, universities and more.
By scholarly literature, Google means publications that are based on the results of research or studies.
Google, on the other hand, has a broader scope, and is looking for resources regardless of where they come from. Resources in a Google search do not have to be scholarly, and do not have to be based on research.
Because much of what ends up in Google Scholar is vetted by a program that Google uses to identify scholarly works (and not a real person), you want to be careful and double-check that what you find is in fact scholarly. While the bulk of it is, there have been cases of non-scholarly content being found in Google Scholar.
Which version of Google should I use when?
Knowing which version of Google to use depends on what it is you are trying to research. For most people starting out with a research question, Google is a good place to start to get an idea of what's been published on your topic, as well as to get valuable background or anecdotal information.
Progressing in your research you may realize that you need some studies or hard data to back up your stance so you may want to switch over to Google Scholar to focus your search on the more scholarly literature.