Short answers, pulled from the story.
Tristan und Isolde premiered on the 10th of June 1865 at the Königliches Hoftheater und Nationaltheater in Munich. Hans von Bülow conducted, with Ludwig Schnorr von Carolsfeld and Malvina Schnorr von Carolsfeld in the title roles.
The Tristan chord is the first chord heard in the prelude to Tristan und Isolde. Rather than resolving to a consonant harmony, it resolves to another dissonant chord. Scholars regard it as a defining moment in the evolution of Western music, marking a departure from traditional tonal harmony and pointing toward the development of atonality in the twentieth century.
Two forces shaped the opera's conception. In October 1854, Wagner read Arthur Schopenhauer's The World as Will and Representation, which triggered what he described as a "serious mood" seeking ecstatic expression. His passionate involvement with Mathilde Wesendonck, wife of his patron Otto Wesendonck, provided the personal emotional fuel, and he wrote to Franz Liszt in December 1854 describing the opera as a memorial to a love that had found "utter repletion."
Despite more than seventy rehearsals held between 1862 and 1864, the Vienna Court Opera could not mount a successful production. The original tenor engaged to sing Tristan, Alois Ander, proved incapable of learning the role. Parallel attempts in Dresden, Weimar, and Prague also failed, and the opera acquired a formal reputation as unperformable before King Ludwig II of Bavaria's patronage finally made the Munich premiere possible.
The opera draws on Schopenhauer's concept of the Will as a restless, never-satisfied force driving human desire, and uses Day and Night as metaphors for the external world of social obligation versus an inner realm of authentic love. It also incorporates elements from Christian mysticism, Vedantic and Buddhist metaphysics, and the Sufi concept of fana, the annihilation of the self in the divine.
The opera directly inspired Anton Bruckner, Gustav Mahler, Richard Strauss, Alban Berg, Arnold Schoenberg, and Benjamin Britten. Claude Debussy, Maurice Ravel, and Igor Stravinsky formulated their own styles partly in contrast to Wagner's legacy, and Schoenberg's Verklärte Nacht of 1899 was described by Aaron Copland as clearly inspired by Tristan.