Google Scholar

Google Scholar is a freely-accessible Web search engine that indexes the full text 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 journals of Europe and America's largest scholarly publishers. It is similar in function to the freely-available Scirus from Elsevier, CiteSeerX, and getCITED. It is also similar to the subscription-based tools, Elsevier's Scopus and Thomson ISI's Web of Science. Its advertising slogan — "Stand on the shoulders of giants" — is a nod to the scholars who have contributed to their fields over the centuries, providing the foundation for new intellectual achievements.
Features and specifications:
Google Scholar allows users to search for digital or physical copies of articles, whether online or in libraries. Google Scholar is relatively quick and easy to use. “Scholarly” searches will appear using the references from “’full-text journal articles, technical reports, preprints, theses, books, and other documents, including selected Web pages that are deemed to be “scholarly. Because most of Google Scholar's search results link directly to commercial journal articles, a majority of the time users will only be able to access a brief summary of the articles topics, and small amounts of important information regarding the article, and possibly will have to pay a fee to access the entire article. Google Scholar is as easy to use as the normal Google web search can be, especially with the helpfulness of the "advanced search" option, which can automatically narrow search results to a specific journal or article. The most relevant results for the searched keywords will be listed first, in order of the authors ranking, the amount 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.
Using its "group of" feature, it shows the available links to journal articles. In the 2005 version, this feature provided a link to both subscription-access versions of an article and to free full-text versions of articles; for most of 2006, it provided links to only the publishers' versions. Since December 2006, it has provided links to both published versions and major open access repositories, but still does not cover those posted on individual faculty web pages; access to such self-archived non-subscription versions is now provided by a link to Google, where one can find such open access articles.
Through its "cited by" feature, Google Scholar provides access to abstracts of articles that have cited the article being viewed. It is this feature in particular that provides the citation indexing previously only found in Scopus and Web of Knowledge. Through its "Related articles" feature, Google Scholar presents a list of closely related articles, ranked primarily by how similar these articles are to the original result, but also taking into account the relevance of each paper.
Criticism:
Some searchers consider Google Scholar of comparable quality and utility to commercial databases, even though its user-interface (UI) is still in beta. The reviews recognize that its "cited by" feature in particular poses serious competition to Scopus and ISI Web of Knowledge, although it generally returns fewer results than subscription services.
A significant problem with Google Scholar is the secrecy about its coverage. Some publishers do not allow it to crawl their journals. Elsevier journals were not included before mid-2007, when Elsevier began to make most of its ScienceDirect content available to Google Scholar and Google's web search. As of February 2008 the absentees still included the most recent years of the American Chemical Society journals. Google Scholar does not publish a list of scientific journals crawled, and the frequency of its updates is unknown. It is therefore impossible to know how current or exhaustive searches are in Google Scholar. Nonetheless, it allows easy access to published articles without the difficulties encountered in some of the most expensive commercial databases.Google Scholar has problems identifying publications on the arXiv preprint server correctly. Interpunctuation characters in titles produce wrong search results, and authors are assigned to wrong papers, which leads to erroneous additional search results. Some search results are even given without any comprehensible reason. Google Scholar was not developed to aid someone who is seeking out intense medical clinical questions, but was developed to share the basics of what the importance of the topic is.