Indicators (measures) which intend to quantify a journal's impact on the research community is one of several tools to identify important journals as well serve as a guide in the choice between a number of potential journals to submit your manuscript to. Evaluation of journals has, for somewhat dubious reasons, sometimes been used to assess an individual article. But of course, it doesn't follow that an article, published in a journal with high impact, should be more cited than an article published in a journal with a lower citation rate.
One must also be cautioned as the indicators are usually not normalized, i.e. a journal cannot be compared with another one if they are not publishing articles within the same field. Furthermore, the measures are designed in such a way that a similar distribution is presupposed between original articles and review articles.
Furthermore, one must be cautious of the fact that many indicators, among them the most commonly used Impact Factor (IF) or Journal Impact Factor (JIF) as it is also called, are not normalized. One journal can not be compared with another one if they are not publishing articles within the same research area. The indicators are also constructed in such a way that does not take into consideration that there is an uneven distribution of original research articles and review articles. Journals that publishes a greater amount of review articles tend to receive higher Impact Factors than journals that don't since this type of articles generally receives more citations than other types of articles.
Another common objection is that different measures can create different ranking orders of the same set of journals. In those cases, one must consider whether the measures are non-robust or measure different aspects of impact. If you don't know what the indicators measure it is difficult to know what they really say about the journal.
In the following sections we present databases that produce the most commonly used journal indicators and some basic explanations about what they intend to measure. Please note that journal indicators from one database can not be compared with the ones from another database, even if the indicator is the same, since the indicator values are dependent on the data contained in the specific database.