Citation databases have been developed for evaluating publications. They enable you to count citations and check, for example, which articles or journals are the most cited ones. Citation databases don't cover all publications. The coverage of different disciplines also varies greatly. We subscribe to Scopus and Web of Science and recommend using them for citation purposes. Google Scholar also attempts to count citations.
Scopus is the largest bibliographic database available, with over 65 million records.
You can view a tutorial on Cited Reference Searching here.
Web of Science is a huge database of records which allows citation searching. You can view lots of tutorials relating to citation searching here.
Google Scholar provides a simple way to broadly search for scholarly literature. Citations are available for results. However, citation information in Scholar needs to be treated with caution. Compared to Scopus and Web of Science, there are limited search features, and variations in how the item is cited which can result in multiple entries for the item cited. Unknown sources are indexed, and data quality is sometimes poor.