What is the difference between an unsound argument and an invalid argument?

Arguments (of the deductive variety) are either VALID or INVALID and SOUND or UNSOUND.

See:
A Little Logic (http://home.wlu.edu/~mahonj/LittleLogic.htm)
unsound argument - Google Search (http://www.google.com/search?q=unsound+argument&sourceid=ie7&rls=com.microsoft:en-US&ie=utf8&oe=utf8)