Impact Factors - From ISI (Institute for Scientific Information a part of Clarivate), are a method for attempting to assess a journal's influence and are more widely used in the sciences and social sciences. They are based on the number of citations articles received in a given journal divided by the total number of articles published in that journal all over a given time period. You can locate impact factors from the Journal Citation Reports which has a link on the Web of Science database home page.
Article Level Metrics - SPARC (Scholarly Publishing and Academic Resources Coalition) - offers an overview and analysis on article level metrics.
Altmetrics - or "alternative metrics" is a new area that uses article level measurement such as downloads, pageviews from publishers, collaborative annotation tools, amount of online discussions and more to assess importance. Resources:
Metrics Toolkit - a toolkit that provides guidance for demonstrating and evaluating claims of research impact. Created by IUPUI.
Impactstory - an open source tools that allows researchers to monitor altmetrics for their publications. A Twitter account or ORCID ID are required to particpate.