Wolf-Rayet stars (or WR stars) are very massive stars, with a surface temperature between 25,000 and 50,000 kelvins, luminosity from 100,000 to 1 million times the Sun's
and a mass from 10 to 50 times the Sun's. The star
Gamma Velorum (in the constellation Vela) is just
such a star.
The stars are named after their discoverers, C. J. Wolf
and Georges Rayet (1839 - 1906), who visually observed them in 1867.
What happens under these conditions of high luminosity and high temperature is that the light from the star itself produces radiation pressure which ejects large quantities of matter from the star. This heated matter can be seen in the spectra of these stars as bright "emission lines", rather than the dark absorption lines that most often are found in stars with the same surface temperatures; the so-called O-type stars.
Wolf-Rayet stars are very rare, we only know about 218 such stars in our own galaxy. However, these stars can appear in areas with massive star formation ("starbursts"), and since these clouds and clusters are quite dense, some Wolf-Rayet stars may be hidden inside, still to be discovered.
Wolf-Rayet stars are probably within a few million years of exploding as a supernova because of their very rapid evolution at such large masses, or if it rotates fast enough, maybe even a hypernova, accompanied by gamma-ray bursts.