Journal Impact refers to the importance of a journal in its field. There are different measures of journal impact, one of which is the Impact Factor.
Impact Factor is calculated based on the number of citations, during a given year, to journal articles published within the previous two years. Of the many ways to calculate Impact Factor, the Thomson-Reuters Impact Factor is probably the most well-known.
Other common journal impact measures are the Eigenfactor, SCImago Journal Ranking (SJR), and Source Normalized Impact per Paper (SNIP).
Learn where to find free journal impact metrics.