Google Scholar includes scholarly articles from a wide variety of sources in all fields of research, all languages, all countries, and over all time periods. Records are crawled from publisher websites, institutional repositories and academics' own web pages. Scholar metrics "summarize recent citations to many publications".
Google Scholar citation counts are very likely to be different to the metrics published in proprietary databases like Web of Science and Scopus, because they are unmoderated and may not distinguish between different versions of the same paper (for instance the published article and the accepted manuscript in a repository).
It is not possible to rank records by citation score in Google Scholar. The free Publish Or Perish software can be used to interrogate Google Scholar search results, to identify:
See also Professor Harzing's Slideshare: Citation Analysis, from Publication to Impact (2011).