Impact factors measure the citation usage of a journal's average article within a certain time period. Essentially: the higher the impact factor, the more prestigious the journal. Within the science research community, authors strive to publish in high impact factor journals to secure their reputations and careers.
Scientific journals possessing the highest impact factors include:
Thomson Scientific's database Web of Knowledge assigns impact factors. For a fuller discussion of their calculation, check out Michigan State's guide.
Visualization of the citation flow for the journal Nature. A cooperation between the Eigenfactor Project and Moritz Stefaner.
Bibliometrics is the general term used for a numeric value used to rank the importance of scholarship. Impact factors are one type of bibliometric - for journals. Others include:
Check out Cold Spring Harbor Lab's guide on bibliometrics for a fuller discussion.
Think you can tell the difference between authentic research and a fabrication? Check out this "scholarly" article that fooled the German press.