Some search engines use the full text of each page to put together an index, others just use keywords. When the user enters a keyword or phrase into a search engine, the terms are checked against the index. Search engines often retrieve a large number of results and many of them may be irrelevant.
Most search engines have an advanced search option that allows the user to refine their search.
Scirus Scirus is a science search engine produced by Elsevier Science. It indexes scientific, technical and medical information, including both free and access controlled resources, for which either a paid subscription or an online registration is required. Scirus also reads and indexes non-text files such as .pdf and postscript. Check the help files for search tips. Where a major search engine has an Australasian version, it has been included in this category and omitted from the general category.