Rutgers Libraries provides a detailed explanation on measuring research impact. The page can be accessed here:
http://www.libraries.rutgers.edu/researchers/research_impact
Are you applying for tenure, promotion or a new job? Do you want to include evidence of the impact of your research? Is there an article that you are interested in finding out who have referred to it? To answer these questions, you can use the tools listed on the right to do a so-called "citations analysis".
Be aware that some authors like doing "self-citations", i.e. the author publishes papers that refer to his/her own previous publications. Please use citation analysis tools with caution.