This site from Artlex provides a thorough definition of the Principle of Design known as Rhythm. There are links to other sources and terms as well as visuals from artists such as Klee, Matisse and Mondrian that show further examples.

Every aspect of working with color in visual art can be found on this page. Comprehensive information ranging from value to color interaction.

A definition of composition with other terms and how they relate to composition. This site shows an example and also explains the terms provided.