What is Google Scholar?
Google Scholar allows you to quickly search across literature from numerous disciplines. Google Scholar includes scholarly articles, theses, books, conference papers, and more from multiple organizations, including professional societies, academic publishers, and universities. Google Scholar will provide links to material that is freely available on the web; for other material it will show at least an abstract.
Why not always use Google Scholar?
While Google Scholar is good for quick searching and verifying citations, the list of publishers and organizations from which it collects materials is proprietary, which means you don't really know what publishers are excluded from your search results. It's also difficult to tell how your results are ranked, and features that allow you to narrow your search are not included.
Google Scholar also lacks the depth that subject-specific databases provide. Remember that most everything in Google Scholar will be included in subscription databases, but not vice versa.