Analysis of the citations to a particular author's work can be one component of evaluating the quality of that person's scholarly writing or the impact they have had on their discipline.
A variety of measures have been developed in this regard including:
Citation analysis as a qualitative measurement should be used cautiously, for the following reasons:
The h-index was developed in 2005 by J.E. Hirsch as a method for quantifying the relative impact of an author by counting the number of citations to his or her publications.
An author has an index of h if there are at most h papers that have been cited at least h times.
A researcher's h-index can be determined by listing their publications in descending order of times cited and counting down the list to the last paper for which the number of times cited exceeds the number of papers counted. Some databases, such as Web of Science, will calculate an h-index based on the number of citations to an author in that database.
Rather than a measure of the average number of citations, which can be skewed by either a single highly-cited article, or many new articles which have not yet been cited, this is thought to provide a measurement that avoids over-emphasizing these extreme cases. However, h-index varies by discipline according to the citation practices of a particular field, and should only be used for relative comparisons within a particular discipline.
Hirsch's original paper on the topic appeared in the Proceedings of the National Academic of Sciences 102(46), 16569-16572.
Each may have its advantages (and disadvantages), so it really is best to compare the results of both... as well as other discipline-specific databases that include citation data.
Anne-Wil Harzig, at the Univ. of Melbourne and creator of Publish or Perish software, does a good analysis of the relative disadvantages of both databases in her web article, Google Scholar: A New Data Source for Citation Analysis.
Other research databases may include citation information and may be searched to determine if there are any additional citing publications for an article. All of these databases only list citations from articles in their database, and will miss citations from journals and other publications not in the database. Because of the better subject coverage of some of these databases than Web of Science, they are good places to search in addition to a WoS search.