A great article on SearchWinSystems, found here,
http://tinyurl.com/deffy 
really explains it all.  

In the book, we call the su command "Super User" and then use it to assume the identity of the "root" user and do powerful system-level activites. However, calling it "Super User" is not really accurate. Really su stands for "Substitute User" and can be used to assume the identity of any user, not just the root user like we do in the book.

This Web site explains su quite clearly: http://www.bellevuelinux.org/su.html. However, here are some other references:

 

http://wiki.linuxquestions.org/wiki/Su

http://www.bellevuelinux.org/su.html

http://www.ss64.com/bash/

http://unixhelp.ed.ac.uk/CGI/man-cgi?su

http://linux.org.mt/article/terminal

 

And, of course, typing

man su  

at the promp will generate the man page for su for your specific distribution.

Thanks for Kurt Hudson (www.hudlogic.com) for the inspiration for this tip.