The citation information in Google Scholar is extracted from the scholarly journal articles within the Scholar database, and from the U.S. patents contained in the Google Patents database (linked below). Users have the option to eliminate the patents as the source of citation data and/or the option to include citations from legal journals and opinions from the federal and state courts. If a publication has been cited by these sources, it will contain a "Cited By Link" in its entry; clicking on that link will display the citing journal articles and patents (and the court opinions, if selected)
How to Find Citation Counts via Google Scholar and "Who is Citing Whom"
Be aware:
Google Scholar Citations provides a simple way for authors to keep track of citations to their articles. You can check who is citing your publications, graph citations over time, and compute several citation metrics. You can also make your profile public, so that it may appear in Google Scholar results when people search for your name.