Preprints are early versions of journal articles before they have gone through the peer-review process and been officially published. As the peer-review process can take over a year, preprints are a good source of very current research. You must evaluate each article very carefully before using. Your evaluation should include:
Peer review is a process in which experts in the field analyze and evaluate a journal article or conference proceeding based on the research methods, research conducted, conclusions and overall quality of the article before it is accepted for publication
E-prints are scientific or technical documents exchanged electronically to facilitate communication of new research among scientists and researchers. E-prints include technical communications and scholarly papers that have been submitted for publication. E-prints may or may not have gone through the peer-review process. Note: There may be multiple versions of an article including retractions, addendum, and comments. Thanks to Khue Duong for this information.