The h-index can measure an author's relative research impact, as opposed to a simple citation count. The h-index is based on the set of the author's most cited publications and the number of cited references those publications have received.
A researcher has an index h if h of his Np publications have at least h citations each, and the other (Np - h) publications have at most h citations each. In other words, an author with an index of h has published h papers each of which has been cited in other papers at least h times. (If you've published 3 papers that received 3 citations each, you have an h-index of 3.)
The h-index is an alternative to the journal impact factor metric.
Some databases like Web of Science: Citation Databases from Thomson will calculate an h-index based on citation counts in that database (which is typically not complete). Harzing's Publish or Perish program calculates the h-index based on Google Scholar entries.