Google Scholar searches many of our publishers' websites, but also these databases:
While Google Scholar covers some of the same sources as our databases, it searches them differently. Google Scholar is more likely to search the full text of articles, and so may pick up more obscure references to a topic. This can be good when you need something very specific. For instance, phytoremediation in "Rhode Island"; a similar search in ScienceDirect only turns up 2 articles, neither of which is specifically about phytoremediation projects in RI.
However, that also means that it is easy to get overwhelmed by Google Scholar results, especially with commonly used search terms. For instance, searching for "environment, Russia, and oil" in Google Scholar brings up many articles on business and economics as well as environmental concerns. ("Environment" can also refer to business environment.) Searching in a subject database like Environment Complete helps eliminate some of the non-environmental articles.