The siddur is the prayer book for weekdays and Sabbaths, and its name, meaning arrangement, records exactly what it did: it fixed the order of a liturgy that was once carried in memory.
From memory to book
Before the siddur existed the liturgy was known and led from memory, and the fixing of it in a book was a gradual and consequential act, the earliest ordered prayer books being compiled in the early medieval period in response to requests for the correct arrangement of the service. The siddur sets out the daily and Sabbath prayers in their sequence, so that any worshipper can follow and any community pray in a shared order, and its arrangement became the framework of Jewish prayer. The word names the ordering itself, which is the thing the book contributed.
Rites and revisions
The siddur is not a single fixed text but exists in the several rites of the Jewish world, the differing customs of the various communities, and it has been revised across the movements of modern Judaism, so the prayer book is one of the places where the divisions and developments of the tradition are visible. To hold a particular siddur is to belong to a particular rite and often a particular movement. The book remains the everyday companion of Jewish prayer, distinct from the machzor used for the festivals, and its arrangement continues to order the daily and weekly liturgy it first set down.