Citation databases are key tools in demonstrating the impact of an individual published paper or a researcher's body of published work. They can be used to demonstrate the impact of an article by the number of times it has been cited since it was published (citing references) and to locate a range of other metrics for a researcher's output.
Scopus and Web of Science are two large citation databases that can provide citing references and citation metrics. Google Scholar also provides a 'cited by' feature and citation metrics.
Both Scopus and Web of Science have related platforms that provide additional metric analysis relevant to both researchers and institutions. Scopus data can be further analysed in SciVal. Web of Science data has further analysis available via both Incites and Essential Science Indicators (ESI).
As no single database will contain complete information on who has cited a particular work, it is advisable to use multiple sources.