Hanken subscribes to Web of Science and Scopus (available through Hanna). Google scholar is freely accessible, but you need to activate the library’s link and log in to Hanna to access licensed materials.
In a citation database, you get information not only about who has cited an article, but also about how many times an author has been cited. You can also list all articles citing the same source.
While all sources of citation data are continually trying to extend their coverage, a single database cannot index all the world's scholarly publications. And coverage in different disciplines is different. For example, the coverage of the arts and humanities may be less comprehensive, especially for books.
Generally, Web of Science and Scopus offer good coverage of sciences and social sciences, with less emphasis on humanities. Google Scholar covers all disciplines equally well, and captures 'grey literature' such as policy papers and technical reports in addition to journals. But the criteria for inclusion in Google Scholar are not wholly transparent.