Journal Impact Factor is a measure of importance for scholarly journals. Eugene Garfield first conceived of the idea of an impact factor in 1955 and created a system to measure it. It is used to determine the impact a particular journal has in a given field of research and also to determine in which journal an author might publish. It is calculated each year by the Institute for Scientific Information (ISI), for those journals which it tracks, and is published in Journal Citation Reports.
Garfield, Eugene. "History and Meaning of the Impact Factor." Journal of the American Medical Association 295, no. 1(2006): 90-93.
Elsevier's journal metrics web page.
In the basic sciences, the social sciences, and the applied sciences, there are two good sources for this:
Impact factors are not collected and compiled so comprehensively in the humanities (although it's important to note that Journal Citation Reports includes, as social sciences, such fields as History, Law, Philosophy of Science, and Women's Studies). Often, the best you can hope for, at least for now, is to find a study or reputable poll that evaluates journal quality on a discipline-by-discipline basis. E.g.:
The databases listed here also provide basic statistics like number of articles published per year, number of citations to the journal each year, and number of references made each year.
Journal Impact Factor
Citations to articles from the most recent two full years, divided by the total number of articles from the most recent two full years. "How much is this journal being cited during the most recent two full years?"
5-Year Journal Impact Factor
Citations to articles from the most recent five full years, divided by the total number of articles from the most recent five full years. "How much is this journal being cited during the most recent five full years?"
Journal Immediacy Index
Citations to articles from the current year, divided by the total number of articles from the current year. "How much is this journal being cited during the current year?"
Journal Cited Half-Life
For the current Journal Citation Reports year, the median age of journal articles cited. "What is the duration of citation to articles in this journal?"
Similar to the 5-Year Journal Impact Factor, but removes journal self-citations. It also, unlike the Journal Citation Reports impact factor, includes both the hard sciences and the social sciences.
The Eigenfactor score divided by the number of articles published in journal. "I know how impactful the journal as a whole is, but what about the average individual article in the journal?"
SJR - SCImage Journal Rank
This metric doesn't consider all citations of equal weight; the prestige of the citing journal is taken into account.
SNIP - Source-Normalized Impact per Paper
SNIP weights citations based on the number of citations in a field. If there are fewer total citations in a research field, then citations are worth more in that field.