Google Scholar is a freely-accessible web search engine that provides a simple way to search for scholarly literature. However, we do not know the full scope of what is covered because the indexing is not published.
Google Scholar and library databases work fairly differently from each other. Most of these differences fall under the scope of the search results adn the options for controlling searches.
Google Scholar may generate many more results than searching library databases but the relevancy of the results may not be as high. The searcher will have to parse out the relevancy of the results. Library databases often focus on certain fields of study and the contents are professionally indexed.
Searching in library databases will allow the researcher to more easily access the full text of scholarly articles that have been purchased by the library.