PubMed comprises more than 20 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full-text content from PubMed Central and publisher web sites.
A web search engine is designed to search for information on the World Wide Web and FTP servers. The search results are generally presented in a list of results and are often called hits. The information may consist of web pages, images, information and other types of files. Some search engines also mine data available in databases or open directories. Unlike Web directories, which are maintained by human editors, search engines operate algorithmically or are a mixture of algorithmic and human input.
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Wikipedia is a Web-based, free-content encyclopedia written collaboratively by volunteers and sponsored by the non-profit Wikimedia Foundation. It contains entries both on traditional encyclopedic topics and on almanac, gazetteer, and current events topics. Its purpose is to create and distribute a free international encyclopedia in as many languages as possible. Wikipedia is the most popular reference site on the internet, receiving tens of millions hits per day.