A digital signature helps to keep a document that is transmitted electronically authentic - meaning
the recipient of the document knows who has created it, and is also assured that there have been no alterations to the document after it left the sender. The digital signature used by the sender of the document `encrypts' or locks the document. He uses keys to encrypt it. Only the authorised recipient, who has the appropriate key to the document, can open it once he receives it. Software takes care of the digital signature processes.
For more information on how a digital signature keeps a document tamper-proof, please look up this URL: http://computer.howstuffworks.com/
question571.htm