ISI is the Institute for Scientific Information, which developed and produced the Science Citation Index (SCI), Social Sciences Citation Index (SSCI), and the Arts and Humanities Citation Index (AHCI). ISI is now owned and operated by Thomson Reuters, which as integrated the three indexes into the “Web of Science,” (WoS). WoS is a citation index that provides citation info for over 12,000 journals in all different fields. WoS lists all bibliographic references in the journals it covers (although it doesn’t include citations in books or in book chapters, with a few exceptions). Just as a point of info, WoS does have a number of problems as a source of citation data, but I’m not sure that matters for you.
Thomson Reuters also operates Journal Citation Reports (JCR), which provides a lot of info, including impact factors, about science and social science journals. JCR has a Science Edition and a Social Science Edition. However, JCR only provides journal level info – it does not have info on citations of individual scholars or articles. For any given year (since 1997, when JCR began), you can search the JCR for a specific journal, or for all the journals within a subject category, or all journals published by a specific publisher, or all journals published in a specific country. For each journal and year, the JCR includes things like the total number of citations in the journal, the journal impact factor (JIF), the 5-year JIF,trend graphs, journal immediacy index, journal cited half-life, and a lot of other stuff. JCR does not cover all the journals in the WoS, though.
Both WoS and JCR are part of Thomson Reuters’ Web of Knowledge platform – Web of Knowledge also includes a number of other resources as well, like Medline and Current Contents.