Physics Classroom
The Physics Classroom: The Nature of a Wave
A very good introduction to the world of waves. Waves and wavelike motion, what is a wave, and categories of waves are the topics examined in this site. Many links to other wave sites.
BBC
Bbc: Gcse Bitesize: General Properties of Waves
Light travels as waves. Waves can be described by their amplitude, wavelength and frequency. The speed of a wave can be calculated from its frequency and wavelength.