Superconductivity the property of a material to induce no resistance on electric currents when they are passed throughout the material. We see that no material is a complete insulator — all materials conduct electricity when cooled down to a certain temperature. So no material is, say, 100 percent resistant, resistance is just a measure of how much a material can conduct at room temperature.
However, there’s a really interesting relation between the increase of temperature and the conductivity of a material. Let’s take gold, for example. Gold is one of the best conductors there is — but it shows more resistance as it is heated. This relation is shown throughout all materials. (Chart at http://physicalsciencenow.files.wordpress.com/2013/02/main-qimg-18228a16a8fc1114d6d0de52dd08c2ba.png)
Now, where does this factor into superconductivity? We see that the cooler we make a material, the more conductive it gets. The physicist Heike Kamerlingh Onnes tested this, and found that when he cooled a wire of Mercury to -269 degrees Celsius, the resistance dissapeared.
The BCS Theory also helps explain this phenomenon. The theoory explains that materials suddenly become extreme conductors when the electrons inside them join forces to make what are called Cooper pairs (or BCS pairs). Normally, the electrons that carry electricity through a material are scattered about by impurities, defects, and vibrations of the material’s crystal lattice (its scaffold-like inner structure). That’s what we know as electrical resistance. But at low temperatures, when the electrons join together in pairs, they can move more freely without being scattered in the same way. 
Not all materials show superconductivity. Apart from mercury, the original superconductor, you can find the effect in about 25 other elements (mostly metals, semimetals, or semiconductors), though it’s also been discovered in thousands of compounds and alloys. Each different material becomes a superconductor at a slightly different temperature.
However, some materials are superconductive at high temperatures. It was thought that materials were only superconductive at low temperatures, but in 1986, two European scientists working for IBM, German physicist J. Georg Bednorz (1950–) and Swiss physicistK. Alex Muller (1927–), discovered a ceramic cuprate (a material containingcopper and oxygen) that could became a superconductor at much higher temperatures 
 Superconductors | Introduction | Causes | High-temperature | Uses