The Hallelujah (Aleluia) is part of the oratorio Messiah, which is the most famous work by the German composer Georg Friedrich Händel. Oratorio is a genre of musical composition basically sung, being widely used, besides the orchestra, vocal solos and choir. It generally narrates biblical themes.
The Messiah (HWV 56, 1741) is an oratorio about the life of Jesus Christ, the Messiah. It has 51 movements divided into three parts: the Prophecies about Jesus' birth, the Passion, and the Resurrection. The Hallelujah is the 42nd movement, at the end of the second part, where all the joy over the Messiah's victory over death and sin is demonstrated.