This site from the Georgia State University contains a good overview of some of the magnetic properties that are common to all materials, such as susceptibility and permeability, and then specific discussion of both diamagnetic and...

This site from the Georgia State University provides a good introduction to and illustration of the Meissner Effect, the property of a superconducting material whereby it actively excludes a magnetic field from within its interior.

Wikipedia.com offers a good introduction to diamagnetism and diamagnetic materials, including discussion of how superconductors exhibit diamagnetic properties.

Wikipedia.com offers a wonderful introduction and explanation of the Meissner effect, explaining how this effect qualifies superconduction as a true thermodynamic state of a material.

This site from the Georgia State University provides a brief introduction to diamagnetism, explaining how Lenz's law accounts for diamagnetic behavior in all materials.