The journal impact factor is based on the number of times that articles in a journal are cited in the two years following the year of publication. The impact factor of a journal is calculated by dividing the number of current year citations to the source items published in that journal during the previous two years.
Journal Citation Reports are part of the Web of Knowledge universe. They provide impact factors and times cited for journals for more than 10,000 of the world's most highly cited, peer reviewed journals in approximately 200 disciplines. High impact factor or highly cited journals are considered more prestigious and important.
Some authors use h-factor or h-index which is an index that attempts to measure both the productivity and impact of the published work of a scholar entire body of research.
Impact factor is only one of the measures to consider when assessing a journal's quality. Other factors to consider include:
* Prestige and reputation of the journal.
* Knowledge of the field and the journals in it.
* Acceptance and rejection rate of the journal (this guide includes suggestions for determining the rates)
* Indexing services covering the journal
* Total distribution of the journal
See this guide for more details on each of these factors.