Citation searching is a way of finding relevant research in a field or subject by looking at what an article has referenced and who has since used that article as a reference. For example, you might find a journal article published in 2017. You can do a citation search to find the articles that the 2017 article has referenced, but also find out if anyone has since referenced the 2017 article.
First you identify a key article, author or book which you are studying or has been referenced in an article you are looking at. By using the title or author's name from that source, you can conduct a citation search in a database. This search will give you a list of other articles that have included the same article, author or book in their own reference lists. This indicates that these articles probably discuss some aspect of the subject you are interested in.
You are constructing a "web of knowledge" for your subject. You will usually notice that useful articles appear in journals seemingly unrelated to your topic.
Choose an article or book that will be the target of your search (in publication for at least one or two years). Then locate a database with a citation index. For example, Web of Science, Google Scholar, Academic Search Complete, ScienceDirect, and Scopus include citation indexes of their own.
Here is an example search from Web of Science using Jones, J.C. (1980) Design Methods: Seeds of Human Futures, 2nd edn, Wiley, as the target book citation:
For a visual guide, see these Web of Science recorded training guides.
Here is an example search from Google Scholar using Jones, J.C. (1980) Design Methods: Seeds of Human Futures, 2nd edn, Wiley, as the target book citation: