This screencast introduces the apostrophe and its three common uses.
Source: Kathryn Reilly
Here are some good internet apostrophe resources for further information:
http://owl.english.purdue.edu/owl/resource/621/01/
http://leo.stcloudstate.edu/punct/apostrophe.html
http://www.makeuseof.com/dir/apostrophe-me-how-to-use-an-apostrophe/
Source: see above