These are used to determine how many times, and in what publications, an article is being cited. It is presumed that if an article is cited often, or is cited in prestigeous journals, then it is likely to be of a relatively high quality. A useful article comparing methods of citation analysis is: Yang, K & Meho, L.I. 2006, 'Citation analysis: a comparison of Google Scholar, Scopus and Web of Science', Proceedings of the American Society for Information Science and Technology , vol. 43, no. 1, pp. 1-15. See also Reference Reviews, Peter's Digital Reference Shelf  for reviews of citation tools.
ISI Web of Science  is the best-known and largest citation database. It contains the electronic versions of not just the Science Citation Index but also the Social Sciences Citation Index and the Arts & Humanities Citation Index. Points to remember:
- its focus is more on science & technology (8368 journals indexed as at August 2011) than on arts & humanities (1650 journals indexed), or social sciences (2978 journals indexed)
- a Cited Reference search allows you to search for articles in the database which cite works by a particular author
- even though the database does not itself index conferences, book chapters, theses or reports, information about these will still be retrieved if they have been cited in an article that has been indexed in the database
- the database has no information on any works that are cited outside the 13000 journals it indexes, such as works cited in books or reports. This is a fairly serious drawback for subjects such as the humanities where books and chapters in edited books are important.
- the recent addition of around 150,000 conference proceedings has greatly increased its relevance for subjects such as Information Technology
- see also the Library handout - Web of Science: Searching Citation Indexes  (MS Word)
Scopus  is produced by Elsevier. This product is very similar to ISI Web of Science. It covers around the same number of journals (18,000) and has about the same number of articles (40 million) and also covers conferences. Its main weakness is gaps in its citation data, especially for articles published before 1996. It is particularly strong in the biological and biomedical sciences.
Generally speaking, Scopus is considerably easier to use than Web of Science.
Scopus has been used to evaluate articles in the Australian Government's The Excellence in Research for Australia (ERA) Initiative  (see also ERA Journal Lists  | ERA Journals by Subject ).
Google Scholar 
Google Scholar  is a more academic version of the search engine Google. You can type in an author's name (eg "A Einstein"), or the title of a published work (eg "Fatal Shore"). Authors and titles should be in quotation marks as shown. Each search result will display the number of times the result has been cited by other entries in Google Scholar.
For researchers who publish in formats other than journal articles (for example in books or book chapters), Google Scholar is one of the few tools, and certainly the easiest to use, that can give good citation data. Publish or Perish  is a free, downloadable software program that retrieves and analyzes academic citations using Google Scholar. The program then processes this data into useful formats.
You do need to be cautious about interpreting your results, however. The 'cited by' entries may be of dubious, unreliable or unimportant value academically, and need to be checked. Moreover there is evidence that Google Scholar's citation results are very unreliable: Peter Jacso has written several critical reviews of Google Scholar as a citation analysis tool .
Many journal databases are value-adding by allowing you to search for articles that cite particular authors or titles. This is useful for subjects such as Business or Humanities which are relatively poorly serviced by Web of Science. Unfortunately, unlike Web of Science, they only find information about citing articles if the work cited is also in the database. Examples of these databases are:
- Business Source Premier (but none of the other EBSCO databases) - use the 'Cited References' link in the database toolbar.
- Science Direct (Elsevier journals) - from within the full text of each article, use the 'Cited By' link in the Actions section.
- Sociological Abstracts (and some other CSA databases such as ASCE Civil Engineering, PsycInfo and the Sage collections) - for each article found, use the 'Cited by' link under the abstract in the results list. Note that this link displays the number of times cited by other articles in the database and will not appear if the number is zero.
- Ovid databases - for each article found after you perform a search, the 'Find Citing Articles' link searches (only) the Journals@Ovid database for citing articles. Note that Journals@Ovid consists almost entirely of full text biomedical and health sciences journals.
- IEEExplore - for each article click on the AbstractPlus link. Citing documents in IEEE or IEE publications are listed at the bottom of the page.
CiteSeer  is a free site from Penn State University and can be used to obtain 'cited by' information. CiteSeer mainly contains references in the Computer and Information Science field, but it also contains many general science and other articles. An equivalent database, BizSeer (formerly SmealSearch) for the academic Business field is now defunct.
There are some ongoing projects in other subject areas by Dr. C. Lee Giles  at Penn State. These use the same search engine, which autonomously gathers articles from online repositories and elsewhere on the web. Citeseer is a useful source for otherwise hard to obtain material such as preprints, white papers and other government and agency reports.
Google Patents 
Google Patents  searches data in the US Patent & Trademark Office (only), giving 'citing' and 'cited by' information for US patents.
Journal Rankings and Impact Factors
Journal Citation Reports (JCR)  - another ISI product, JCR gives whole-of-journal analysis rather than individual articles.
Amongst other information, JCR provides the Impact Factors for over 7,600 selected journals. The impact factor is a measure of how much a journal's articles are being cited elsewhere (weighted by the total number of articles it publishes). It is presumed that a journal whose articles are frequently cited in other journals, is likely to have a relatively high quality or academic value.
Because impact factors differ greatly from one discipline to another it is often useful to use JCR to view a group of journals by Subject Category. This allows you to compare impact factors of journals within the same broad discipline area. The Library has access to JCR back to 1999 so impact factors can be determined back to this year.
SCImago  is a free journal analysis tool from a group of Spanish universities. It uses data from Scopus in much the same way as JCR uses data from Web of Science, to produce lists of journals in various subject areas. These can be ranked in various ways such as by number of highly cited articles. SCImago can also be used for comparing countries' research outputs.
Journal Quality List  compiled by Anne-Wil Harzing at the University of Melbourne, this gives rankings for 910 mostly business-related journals from 18 different sources. The author also has a link to her presentation on journal rankings.
The software Publish or Perish  can be downloaded for free from this site.