At one time, only the Web of Science database collected citation count data. Originally, Science Citation Index, as it was known when first published in print in the early- to mid-1970s, focused on the article-centric science literature but now has expanded to almost every subject area. In addition, citation count data is found in many indexing or abstracting services, not just Web of Science. What was once a unique feature is now common but therein lies some issues of concern.
Data contained in Experts.ASU is derived from Elsevier's Scopus database. As the name implies, only data for ASU faculty is available; for articles not in Experts.ASU, use the Scopus database to obtain a citation count. Because the data comes from the same source, you may compare citation counts from Experts.ASU with citations counts from Scopus (provided the articles are in the same field and published close to the same date.)
To find citation counts in Google Scholar:
To find citation counts in Scopus
In addition to the two large multidisciplinary databases developed for citation analysis (Web of Science and Scopus), many other indexing and abstracting service now also provide citation counts for the articles in their databases. These other databases have limitations, however, mainly because they do not have as wide a scope of sources from which they pull the data and for the lack of specific citation reports the larger databases can generate. Frequently, citation counts from subject-focused databases will be smaller than the counts obtained from Web of Science, Scopus or Google Scholar.
For example, while PubMed provides citation counts for articles, it is only looking at the other articles in the PubMed database to obtain that count. Should an article in PubMed be cited by a social sciences journal article, PubMed will not be able to count that citation.
On the other hand, these specialized subject databases may cover some lesser-known journals in the field or even cover types of literature (patents, conference papers) that the larger services don't. If your goal is to find as much literature as you can that has cited an article then always include searching the specialized subject databases in your area in addition to the large multi-disciplinary ones.
As always, make "apples to apples" comparisons; if you use citation counts from a subject focused service, only compare them with counts from the same service.
Most citation counts in other databases will either be located on each entry in the results list (similar to Web of Science or Scopus) or you may have to click on the title in each entry to find the count on the "full record" page.