The machzor is the prayer book for the festivals, and above all for the New Year and the Day of Atonement, whose services are far longer and more elaborate than those of ordinary days.
For the festivals
The word means cycle, and the machzor gathers the liturgy of the festival cycle, distinct from the siddur of the everyday and Sabbath prayers, because the services of the great days differ so much in length and content that they require a book of their own. The services of the New Year and the Day of Atonement in particular are long and dense, and the machzor for those days is correspondingly large, holding the special prayers, confessions and themes proper to the season of judgement and atonement.
The piyyutim
What most distinguishes the machzor is its great body of liturgical poetry, the piyyutim, elaborate poems composed across many centuries and inserted into the festival services, which give the liturgy of the great days much of its particular character and difficulty. These poems, often intricate in language and allusion, are one of the treasures and one of the challenges of the festival prayer, and the differing selections of them are among the marks of the various rites. The machzor thus carries not only the structure of the festival services but a poetic tradition that the ordinary daily prayer book does not, making it the book of the most solemn and most crowded days of the Jewish year.