Web of science and Scopus also provide the citations.Depending on the discipline and cited article, Google Scholar may find more cited references than Web of Science or Scopus because overall, Google Scholar is indexing more journals and more publication types than other databases.
Web of Science provides citation counts for articles indexed within it. It indexes over 10,000 journals in the arts, humanities, sciences, and social sciences.
Scopus provide citation counts for articles indexed within it (limited to article written in 1996 and after). It indexes over 15,000 journals from over 4,000 international publishers across disciplines.
There are also databases containing limited citation counts:
CSA Illumina Databases (BioOne Abstracts and Indexes, ERIC, PsycInfo, Social Services Abstracts, Sociological Abstracts, Worldwide Political Science Abstracts)
It is properly explained by Professor Behrouz. However, there are many citations from the papers which those journals are not indexed by Web of science or Scopus. Therefore, the number citation you will get in Google citation will be more.