There are different metrics that can be used to rank journals. One widely used is the Impact Factor that measures the frequency with which the average article in a journal has been cited in a particular year.
For most metrics, an individual measure doesn't mean much, but can be used in comparison.
Metrics of any kind can be controversial. If you'd like to read more about the issues here are some starting points.
Want to expand your list of possibilities? Ulrich's is a great place to look or there may be directories for journals in specific fields.
Find out who has cited a known author or work (article, book, conference proceeding, patent, etc).
The h-index is an index that attempts to measure both productivity and impact. The index is based on a set of most cited papers (from an author or it can be calculated for a journal) and the number of citations that those publications have received in other people's publications.