Other
Mml: Notation Module: Clef
Although very simple in its design, this short piece gives a nice overview of musical terms, one of which is the treble clef. The author briefly discusses the attribute of "key" in conjunction with the clef.
Other
Music Theory First Aid for Struggling Students
This site provides music theory information on such topics as rhythm, intervals, scales, chords, etc. Also, there is a fine section which describes the basic concepts of form and structure.
Music Theory
Music theory.net
An online beginning music theory and ear training site. This site is for students to use at home to reinforce what they learn in class.