In every discipline there are journals that scholars consider highly influential. Journals are judged by various metrics, including some that assess the "impact" of published articles. These metrics usually look at the number of cited references made to papers published in a given journal.
Citation metrics have received some criticism because they are not a clear measure of quality. (For more details see the Declaration on Research Assessment and the European Association of Science Editors Statement on Inappropriate Use of Impact Factors.)
In the library database Journal Citation Reports (JCR) (provided via ISI), search for a specific journal to see its impact factor, or view journals in a specific discipline/subject category ranked by impact factor. Choose the JCR Year you are interested in, or it defaults to the most current year. Then you can sort journals in a specific subject ranked by impact factor, eigenfactor, or total cites.
More on how impact factor is calculated (article from Journal Citation Reports)
Eigenfactor looks at the impact of journals that cite each article. While available via JCR, they also have a website where you can search for a specific journal, search by discipline (choose an ISI (aka JCR) Category), or search by year.
