What is a Wolf-Rayet Star?
A Wolf-Rayet star is a massive star in the final act of its life, so hot and so luminous that radiation pressure is tearing its own outer layers off. The hydrogen envelope is already gone; what you are looking at is the exposed, fusing core of a star, burning at up to 210,000 K.
The wind is the defining feature. A Wolf-Rayet star loses on the order of 10^-5 solar masses every year — a hundred million times the Sun's rate — driven outward at 1,000 to 3,000 kilometres per second. Over a few hundred thousand years that strips away everything above the core, and the star ends up a bare ball of helium, carbon or oxygen, still fusing furiously.
That is also why their spectra look wrong. Charles Wolf and Georges Rayet found the first three in 1867 at the Paris Observatory and could not identify the broad, bright emission lines they saw. The explanation took decades: we are not looking at a stellar surface at all but into a thick outflowing wind, and the lines are broadened by its speed. The subclasses track what is now exposed — nitrogen for WN, carbon for WC, oxygen for the rare and extreme WO.
They do not end quietly. Having shed its hydrogen, a Wolf-Rayet star detonates as a stripped-envelope Type Ib or Ic supernova, and the fastest-rotating ones are the leading candidates for long gamma-ray bursts — the most energetic explosions since the Big Bang. Some are spectacular in advance: WR 104 pairs with a companion to sculpt its wind into a rotating dust pinwheel visible from 8,000 light-years away.