Google Scholar is a freely accessible web search engine that indexes the full text or metadata of scholarly literature across an array of publishing formats and disciplines.

Released in beta in November 2004, the Google Scholar index includes most peer-reviewed online academic journals and books, conference papers, theses and dissertations, preprints, abstracts, technical reports, and other scholarly literature, including court opinions and patents.

Google Scholar uses a web crawler, or web robot, to identify files for inclusion in the search results. For content to be indexed in Google Scholar, it must meet certain specified criteria. An earlier statistical estimate published in PLOS ONE using a Mark and recapture method estimated approximately 80–90% coverage of all articles published in English with an estimate of 100 million (The Number of Scholarly Documents on the Public Web). This estimate also determined how many documents were freely available on the internet.

Google Scholar has been criticized for not vetting journals and for including predatory journals in its index.

The University of Michigan Library and other libraries whose collections Google scanned for Google Books and Google Scholar retained copies of the scans and have used them to create the HathiTrust Digital Library.

Google Scholar allows users to search for digital or physical copies of articles, whether online or in libraries. It indexes "full-text journal articles, technical reports, preprints, theses, books, and other documents, including selected Web pages that are deemed to be 'scholarly.'"

Because many of Google Scholar's search results link to commercial journal articles, most people will be able to access only an abstract and the citation details of an article, and have to pay a fee to access the entire article.

The most relevant results for the searched keywords will be listed first, in order of the author's ranking, the number of references that are linked to it and their relevance to other scholarly literature, and the ranking of the publication that the journal appears in.

 

Note:   The above text is excerpted from the Wikipedia article Google Scholar, which has been released under the GNU Free Documentation License.