These two citation databases are powerful research tools that cover a vast scope of interdisciplinary subject matter involving all topic areas. Both also include "times cited" links that allow a researcher or author to follow-up on subsequent citation patterns for an article(s) of interest.
Web of Science - Is not limited to the sciences. Some may be familiar with this under names like "social sciences citation index," "arts & humanities citation index," et al. Begin with a relevant article about one's topic and then quickly uncover subsequent authors who cite that work in their own publications. But BEWARE, WofS can be uneven in coverage of some disciplines, and is not a primary tool for zeroing in on discussion of communications, journalism or media if that discussion appears in non-scholarly/trade publications.
Scopus - Scopus is the world's largest abstract and citation database, with over 33 million records. Its coverage of Scientific, Technical, Medical and Social Sciences literature includes 15,000 peer-reviewed journals, 1,200 open-access journals, 500 conference proceedings, over 600 trade publications, and 200 book series. Scopus also covers 386 million quality web sources, including 22 million patents.