Google Scholar uses search robots to generally index every paper from every website they visit, including most major sources and also many lesser known ones.
It is primarily a search of academic papers. Shorter articles, such as book reviews, news sections, editorials, announcements and letters, may or may not be included. Untitled documents and documents without authors are usually not included. Website URLs that aren't available to their search robots or to the majority of web users are not included either. They also do not include websites that require you to sign up for an account, install a browser plugin, etc.