Always consider metrics in the context of your discipline. Citation patterns vary in different disciplines and metrics should never be used as the only measure.
"The index h, defined as the number of papers with citation number greater than or equal to h, is a useful index to characterise the scientific output of a researcher". (Hirsch, 2005)
The example below is an Author Citation h-Index from Scopus, where the author has 16 documents that have been cited at least 20 times by other researchers.
The h-index, will vary depending on the journals indexed by the selected source. The source of the data should therefor be quoted when citing author impact. Google Scholar usually have a higher h-index than WoS and Scopus.