1) It was composed in 1741 by George Frideric Handel.
2) Handel tweaked his "Messiah" many times through the years, and different versions get performed; the cognoscenti like to keep track. There's the 1741 version from Handel's autograph score; there's the 1742 version from the oratorio's Dublin premiere; there's the 1743 version from its London premiere. And so on.
3) The oratorio follows the liturgical story line: the coming of Christ, the resurrection, the ascension, the glorification.
4) Its most famous passage is the "Hallelujah" chorus. If you don't already know that ...
5) Less famous is "the trumpet shall sound," oozing mystery, as the specter of Judgment Day is evoked.