The Georgia State University Library has a nice page to teach you how to use Google Scholar here.
You can use it to search for scholarly materials (journal articles, books, theses, preprints, etc.) from "a wide variety of academic publishers, professional societies, preprint repositories and universities, as well as scholarly articles available across the web." Doing a few random searches, I found content from PubMed and PubMed Central, Astrophysics Data System, ACM, IEEE, Springer, Wiley, Elsevier, American Physical Society, American Institute of Physics as well as Science, Nature, Genetics, Plant Physiology, Clinical Chemistry, and Journal of Biological Chemistry. For books, you can connect to OCLC's Open WorldCat to see if libraries in your area (including GSU) own them.
As a searching tool, Google Scholar is still extremely basic. You cannot limit searches by date, language or format (to journal articles only, for example). You cannot build complex searches or combine previous searches. The search results, ranked by "relevancy," cannot be re-sorted by date. There is no automatic mapping between synonyms; compare the results for "heart attack" with "myocardial infarction." There is also no "related records" search option except for looking at articles that cite your results, which is not very precise.