After writing so much about my trawl through Web of Science, I was curious to learn a little more about it.
Web of Science Core Collection covers over 12,000 journals worldwide across more than 250 disciplines. Coverage is current and retrospective, sometimes dating back all the way to 1900. Journals covered are both open-access and for-profit. The database also contains more than 160,000 conference proceedings. It is owned and run by Thomson Reuters, who (as they are quick to point out) do not themselves publish content. That would explain why they are willing to include open access content (which I thought was strange in an earlier post).
The database is composed of a series of separate citation indices. These include Science Citation Index Expanded, Social Sciences Citation Index, Arts & Humanities Citation Index, Index Chemicus, Current Chemical Reactions, Book Citation Index, and Conference Proceedings Index.
All of these combine to make a very wide ranging and valuable interdisciplinary citation index.