Wolf-Rayet stars (sometimes abbreviated as WR stars) are old, very massive stars (~20 solar masses) in the process of ejecting their material into the interstellar medium. In contrast with the Sun, which ejects 10-14% of its mass per year through solar wind, Wolf-Rayet stars lose 10-5 of their mass per year. Their surfaces are extremely hot — 25,000 to 50,000 K.
Wolf-Rayet stars have used up most of their nuclear fuel, being forced to fuse heavier elements such as helium and nitrogen ("WN" sequence) or helium, carbon, and oxygen ("WC" sequence). These heavy elements are kicked up around the atmosphere of the star, and they absorb the light and heat given off by the star, being propelled outwards. Wolf-Rayet stars have atmospheres so large that their thickness is comparable to the star itself.
The large quantity of superheated material being ejected gives WR stars strong emissions spectra, so notable that Wolf-Rayets are among the easiest stars to observe in adjacent galaxies. 230 Wolf-Rayets have been observed in the Milky Way Galaxy, about 100 in the Large Magellanic Cloud, and 12 in the Small Magellanic Cloud.
Wolf-Rayets are relatively short-lived as they cannot continue ejecting major portions of their mass while continuing to survive. Eventually, the star loses the pressure necessary to derive fusion energy from its contents, and the star collapses, forming a collapsar. The core becomes a black hole, while the material in the mantle gets drawn in to the center at relativistic speeds, releasing tremendous amounts of gamma rays. The subsequent explosion is thought to be the physical origin of so-called "long gamma bursts" which last about two seconds.
About 59% of Wolf-Rayets have smaller companion stars, which they interact with throughout their evolution. When a Wolf-Rayet explodes as a supernova, they can send nearby stars flying, getting ejected at supersonic speeds relative to the interstellar medium. Such stars are called runaway stars.