Google Scholar is a Web search engine that specifically searches scholarly literature and academic resources. Plain ol' Google searches EVERYTHING, reliable or not.
How does it differ from the Library databases?
There will be overlap between the library's databases and Google Scholar, but not everything in the library can be found in Google Scholar. Sometimes there are links to the fulltext of articles from Google Scholar, but not to everything to which the library has access. It's a convenient way to start, but it shouldn't be your only source of articles.
How do I use Google Scholar?
If you're going to use Google Scholar, be sure to use the university's VPN (Virtual Private Network) or go through the Libraries' website. You will be able to reach some fulltext articles with either of these methods.