The impact factor is a bibliometric tool used to estimate the importance of scientific journals. It is calculated and published annually for journals indexed by the Institute for Scientific Information (ISI) and is a reflex of the average number of citations that each journal receives during a certain period of time. Since its creation it has been used as criteria in several decision-making processes such as buying journals subscriptions and submitting articles for publication. Operating under the hypothesis "the greater the impact factor, the greater the quality of a journal", it has been subject of numerous controversies, especially due to certain biases around its calculation. The present article describes the Impact Factor and analyses the most important aspects in relation to its calculation, strengths, limitations and common errors on its application.