University of Oregon
Determining Properties of Stars
An excellent website detailing how a star's properties can be determined, and how these properties lead to the way a star is classified.
NASA
Nasa Star Child: Stars
This page details the life cycle of a star, and includes information on medium-sized and massive stars, plus a glossary and a question.
Other
Sky Server: Color
SkyServer answers why stars have different colors and shows what you can learn by studying the colors of stars, galaxies, and other objects.