The following description is taken from Sam Morgenstern, Composers on Music (Pantheon, 1956), p. 377.
Vienna, cradle of some of the most eventful movements in music history, witnessed the inception of Schoenberg’s twelve tone technique. The father of this theory and practice began his career as a romantic at the tail end of the Wagnerian hegemony. His most frequently played piece, VerkUirte Nacht (1 899), carries Tristan to its ultimate conclusions. It is interesting, in view of Schoenberg’s lifelong artistic battles, that one of the Vienna Tonkunstlerverein jury which refused performance to the work remarked of it: "Das klingt ja, als ob man uber die noch nasse Tristan Partitur daruber gewischt hatte" (It sounds as if one had smeared over the still moist Tristan score).
After passing through Atonalism, Impressionism and Expressionism (Pierrot lunaire  is the high point of the last), Schoenberg evolved his twelve tone technique. Uncompromising, in spite of the perpetual critical opprobrium heaped upon him, he carried it to its highest development. In his last works, however, he included some tonal writing.
Practically an autodidact except for some formal lessons in counterpoint with Alexander von Zemlinsky, Schoenberg was a great teacher; consequently his enlightening exposition of twelve tone technique is of paramount importance. His essays on Brahms and Mahler, whom he greatly admired, are novel and penetrating, and his Harmonielebre (1911) is one of the definitive textbooks on modern music theory.
A group worthy of emulation was the Society for Private Musical Performances which Schoenberg founded and directed, in which modern works of all trends were played before chosen audiences after painstakingly careful preparation. Only so might new works receive considered and just criticism.
click here to read some of Schoenberg's remarks about his own music