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.