An article's accumulated citations can be used to measure the degree to which the academic research community has been influenced by, or reacted to a publication. It is sometimes difficult to compare citation counts between disciplines as some disciplines cite highly while others don't. The SJR metric in Scopus looks at both the quality of the citing journal and the citations and is a normalised metric as it allows comparison between disciplines.
One thing to keep in mind when looking at an article's citation count is that the number of citations will vary across different databases as each database is drawing citation information from its own different pool of titles.
Citation databases Such as Scopus and Web of Science provide citation behaviour and counts amongst the titles they index (known titles).
Individual databases which currently provide citation information for articles are using citation information from within their database or within their suite of databases (known titles)
Google Scholar on the other hand provides citation data from a basically unknown list of titles.
Alternative metrics track social media buzz, usage, downloads etc and provide another way for researchers to identify how other researchers and readers feel about their article. More information is available under the tab Alternative metrics on the left of this screen.