Citation analysis involves examining an item's referring documents (citations). It is used in searching for materials and analyzing their merit.
Data from citation indexes can be analyzed to determine the popularity and impact of specific articles, authors, and publications. Using citation analysis to gauge the importance of one's work, for example, is a significant part of the tenure review process. Information scientists also use citation analysis to quantitatively assess the core journal titles and watershed publications in particular disciplines; interrelationships between authors from different institutions and schools of thought; and related data about the sociology of academia. Data from citation analysis is also useful in assessing the impact or possible effects of publications existing in electronic formats (i.e., are online or free journals cited more than others).
Although citation analysis is nothing new (the Science Citation Index began publication in 1961), greater computing power is making it more useful and widespread. Google's PageRank is based on the principle of citation analysis.
Link popularity isn't the only criterion that Google measures when ranking search results. The popularity of the referring pages is also a factor (via a hubs and authorities algorithm), as of course is the contents of the actual page. Web pages that contain similar links, and pages that are both linked to from other pages, have a greater chance of appearing together in Google's "similar pages" feature. Also, PageRank is not perfect, nor are Google's results. Google Bombs, for example, can also throw off accurate search results. Yet Google's ranking methods have proven more reliable than solely counting on possibly spammed text and keyword jamming.
An excellent and recent review of citation analysis as applied to journal articles can be found at: White, H. D. (2004). Citation analysis and discourse analysis revisited. Applied Linguistics, 25(1), 89-116.
See also: Semantic Web