Measuring the "impact factor" is a way to determine the relative importance of a journal within its field. The higher the impact factor, the more important the journal is compared to others in the same discipline. Impact factors are determined based on calculating the number of citations to articles published in a particular journal. This practice is most commonly used in science/technology and social sciences fields.
For more information on Impact Factors see the Libraries' Reseach Impacts Using Citation Metrics guide.
Few tools exist to measure impact factors for Humanities journals. Tools like ISI Journal Citation Reports and Eigenfactor focus on journals in the fields of science, technology, and social sciences.
One way to assess the impact of a published book or journal article is to determine how many times it has been cited by others. The more times an article or book has been cited, the higher its impact. Google Scholar is one tool to find cited references.
"Cited reference searching" refers to the practice of using an important article on a research topic to find later articles that cite the original work. These references usually deal with the same topic and report results of later research on the subject. Cited reference searching can be a very effective method of tracking research in a given field and of finding highly relevant references to the topic of interest.