http://micro.magnet.fsu.edu/micro/gallery/superconductor/super.html (Ceramic Superconductor Single Crystal)
As most people know (or don't know, whichever is the case) the component of an electrical circuit that causes energy loss is called "resistance," which can be defined as a materials opposition to current being passed through it. Usually, this resistance results in the production of heat, sound, or another form of energy. In many cases, this transformation of energy is useful in such applications as toasters, heaters, and light bulbs. Even though it is a useful property, resistance often gets in the way of performance in such cases as high voltage transmission wires, electric motor output, and other cases where internal system energy losses are unwanted. This is where the phenomenon of superconducting materials comes into play and may present the solution to this energy loss problem.
Superconductors are materials that display zero resistance under certain conditions. These conditions are called the "critical temperature" and "critical field," denoted Tc and Hc respectively. The Tc is the highest temperature state the material can attain and remain superconductive. The Hc is the highest magnetic field the material can be exposed to before reverting to its normal magnetic state. Within the substances currently known to superconduct, there is a divide between what has come to be called type I and type II superconductors. Type I are composed of pure substances, usually metals, and type II are composite compounds, usually some sort of ceramic.
Additional differences between type I and type II exist, mainly that type II display superconducting qualities at much higher temperatures and can remain superconductive in the presence of much higher magnetic fields. While type I have Tc's that hover just a few degrees from absolute zero, type II can have Tc's of over 130 K. The graph below shows how type I and type II superconductors compare as related to Tc and Hc:
http://www.americanmagnetics.com/tutorial/supercon.html
The difference in magnetic fields is also quite large. Type I superconductors can stand fields up to approximately 2000 Gauss, which translates to about .2 Tesla, while type II can with stand fields of up to several hundred thousand Gauss, which translates to more than 10 Tesla. The magnetic field for any temperature below the Tc is given by the follwoing eqution:
Bc ≈ Bc(0) * [1 - (T/Tc)^2]