This screencast defines passive voice and provides examples.
Source: Kathryn Reilly
The following websites provide additional information on the passive voice:
http://grammar.ccc.commnet.edu/grammar/passive.htm
http://www.englishclub.com/grammar/verbs-voice_passive.htm
http://esl.about.com/od/grammarstructures/a/passive_voice.htm
Source: see above