A digital signature is an electronic sequence of bits that stands as a unique demonstration of its user’s identity. For more details on the term, see the excellent definition at WhatIs.com: http://searchsecurity.techtarget.com/sDefinition/0,,sid14_gci211953,00.html. For a more detailed tutorial on the subject, see the American Bar Association’s coverage of digital signatures at http://www.abanet.org/scitech/ec/isc/dsg-tutorial.html.
www.searchsecurity.com will provide pointers to plenty of commercial implementations you can use as drop-ins into existing or new code (but these do require licensing fees).
Open source implementations aren’t extremely numerous, but you can find them through the GNU organization (GNU Privacy guard at www.gnupg.org) as well as various implementations of XML security markup languages, such as the XML-Security stuff available through the Apache XML Project at http://xml.apache.org/.
HTH, and good luck with your programming efforts.