The traditional view of impact is based on the journal impact factor (JCR) developed by the folks at the Web of Science database. Journals whose articles are more quickly and more often cited have a higher impact factor. PSU no longer subscribes to the JCR database, but you can access the JCR database (also called InCite) in Portland at the OHSU library and the UO library in the White Stag building.
There are alternative ways of assessing impact:
There are other ways to measure impact and many progressive Promotion and Tenure committees value alternatives to the Journal Citation Report method above.
Read an interesting article on alternative metrics - From Bibliometrics to Altmetrics: A Changing Scholarly Landscape.