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.
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.
Open Curriculum
Open Curriculum: Pitch: Sharp, Flat, and Natural Notes
To understand the definition of a pitch, its effect on notes, and the associated symbols.
Other
Music Concepts Online: Notation: Basic Rhythm and Pitch
This site looks at rhythm, terms and concepts, which include rhythm, score, note (parts of and kinds), rests, tie. Great information.