Journal Citation Reports (JCR) contains a wide variety of citation related data for almost 13,000 academic journals. You can search in the collection either by 1) journal title or by 2) research discipline. JCR divides journals into 235+ research discipline collections, but there is substantial overlap between these collections.
1) Journal title: Each entry contains a variety of citation related data on the journal. The most frequently used data is the 2-year (or 5-year) impact factor. The impact factor = # of citations to the articles journal from the last 2 years / the #of articles published in the journal in the last two years. Typically impact factors range from 0.5 to 50+, but this is very specifically discipline related with substantial variation between research areas. Because of this factor you will usually want to look at the journals within your research area by performing a research discipline search.
2) Research discipline: JCR divides journals into 235+ research discipline collections. Since there is overlap between these collections you can select multiple areas for your search. Your results list can then be sorted by a wide range of citation related statistical measures so you can see the most heavily 'used' journals in your area of research. The specific citation patterns in your research area should be strongly considered when analyzing this data; there is no 'magic number'! And remember that the JCR collection is already selected with an eye to containing the most 'heavily cited/important' journals in academia. Just being contained in this list is a measure of quality.
Link to Journal Citation Reports.