What are superconductors?

"A superconductor is any material that can conduct electricity with no resistance."
https://www.techopedia.com/definition/9582/superconductor

"Superconductivity is a phenomenon whereby a charge moves through a material without resistance.

In theory this allows electrical energy to be transferred between two points with perfect efficiency, losing nothing to heat"
https://www.sciencealert.com/superconductivity

"The special "talent" of superconductors is that they have zero resistance to electric current. Absolutely none. In theory, a loop of HTS wire could carry a circling current forever without even needing a power source to keep it going."
https://www.nasa.gov/audience/forstudents/9-12/features/superconductor_feature.html

"The answer lies in the incredible property of superconductivity, a unique material characteristic that has the potential to revolutionize electrical transmission, transportation, and physics as we know it."

"That is what makes superconductivity so special. Superconductivity is when a material stops resisting an electric current and allows it to pass through it freely, without any apparent energy loss as a result.

To get material into a superconductive state, the material has to be frozen to an extremely low temperature, sometimes to only a few degrees above absolute zero (-459.67 degrees Fahrenheit, -273.15 degrees Celsius). Then, for reasons that we still cannot explain, electrical resistance abruptly stops, and an electrical current can continue around a circuit seemingly forever."
https://interestingengineering.com/superconductivity-what-is-it-and-why-it-matters-to-our-future

"Enter superconductors. If the three laws of thermodynamics say that there's no such thing as a free lunch, then superconductors have their cake and eat it, too. Send current through a superconducting wire, and it loses no energy to resistance. Bend the wire into a loop, and it will hold charge indefinitely. Levitate it above a magnet, and the sun will devour the Earth before it will fall.

Soon after its discovery in 1911 by Dutch physicist Heike Kamerlingh Onnes and his collaborators, Cornelis Dorsman, Gerrit Jan Flim and Gilles Holst, superconductivity inspired dreams of no-loss electrical transmission. Unfortunately, there was a catch.

Superconductors require very cold temperatures, on the order of 39 kelvins (minus 234 C, minus 389 F) for conventional superconductors. The solid mercury wire that Kamerlingh Onnes used required temperatures below 4.2 K (minus 269.0 C, minus 452.1 F). Even so-called high-temperature superconductors only work their magic below 130 K (minus 143 C, minus 225.7 F)."
https://science.howstuffworks.com/environmental/energy/superconductivity.htm