In his lifetime, Mahler was one of the most prominent conductors in the world. He worked with the Vienna Philharmonic, the Vienna State Opera and the New York Philharmonic. He also composed nine symphonies, several major works for orchestra and voice, and a Tenth symphony that exists in various "completed" versions.
Mahler's symphonies fall into several groups. Nos. 1-4 are all based (to one degree or another) on a song cycle, Der Knaben Wunderhorn. The First, ("Titan")has a conventional four-movement structure, but that's only because Mahler cut one of the original movements. The Second ("Resurrection") adds a chorus to depict the dead rising from their graves. But that's nothing on the Third: where Mahler attempts to capture the totality of nature and God in six movements, lasting 100 minutes. Mahler was nothing if not ambitious.
Nos. 5-7 are all instrumental. The Fifth is a huge, heaven-storming work. The Sixth ("Tragic") shows a hero marching to his doom, felled by the giant blows of a hammer. And the Seventh is a powerful five-movement work that contrasts the mysteries of night with the raw light of day in its final movement.
Mahler finished out his composing career with several monumental works. The Eighth ("Symphony of a Thousand") is two movements and requires a full orchestra, three choruses and vocal soloists. It is a large-scale setting of a medieval hymn ("Veni, creator spiritus") and the final scene of Part II of Goethe's Faust. The title comes from the promoter, who claimed that a thousand performers were required to perform the Eighth.
Having stormed the heavens, Mahler turned inward for his final works. Das Lied von der Erde ("The Song of the Earth") is a setting of Chinese poems for two soloists and orchestra. A symphony in all but name, since Mahler wanted to avoid the "curse of the Ninth" that had prevented other composers (Brahms, Bruckner, Dvorak) from writing more than nine symphonies. Faced with health problems (including an irregular heartbeat) he wrote his Ninth proper, which is based around a faltering rhythm that is a transcription of his own failing heart. He died while working on the Tenth.
Mahler died at the age of 50, having finished his career in New York, running both the New York Philharmonic and the Met. He said of his music that his time would come. With its turbulent emotions, grinning, wry humor and cries from the heart, the symphonies of Mahler are a soundtrack of the the last century, and remain as relevant today as they were when he was alive.