Web of Science curates a citation index of the most respected peer-reviewed research across time, subjects and languages. All subject areas are covered, but particularly the pure sciences. The fact that WoS indexes 20,000 journals in humanities and social sciences as well is indicative of its size and diversity. 

WoS is easy to use for basic searches, but it has many advanced features that make it an excellent tool for the expert researcher as well. For example, you can search big data sets in the social sciences, use the "countries and territories" search to focus on authors' geographic locations, use the "cited reference" search to find articles that have cited a previously published work, and check the "usage count" to see how many other users have clicked on an article. 

You can also search in these databases, selecting all or only one:

• Arts & Humanities Citation Index
• Social Sciences Citation Index
• Science Citation Index
• BIOSIS (the leading biological sciences database)
• Current Contents (current awareness database covering more than 7,000 journals)
• Derwent Innovation Index (patent searching)
• Medline (medical database similar to PubMed)
• Zoological Record (the world's oldest continuing database of animal biology)