A siddur (seh-DOOR or SID-der) is a Jewish prayer book. It is an anthology of prayers, readings, and poetry, some of which date to the time of the Temple in Jerusalem.
The word siddur means “order.” It is just that: it gives the proper order for the service. The plural is siddurim.
There have been many different siddurim since medieval times because each siddur reflects the custom of a particular group of Jews. There are some major, well-known siddurim with wide distribution, such as Mishkan T’filah (Reform), Siddur Sim Shalom (Conservative), Kol HaNeshamah (Reconstructionist), Siddur HaShalem and Siddur Rinat Yisrael (Orthodox.)
Some smaller communities produce their own siddurim. For instance, Congregation Sha’ar Zahav in San Francisco has published its prayer book, Siddur Sha’ar Zahav.
What prayer book is best for you? The one your community uses. While the basic order of service is the same in every siddur, small differences in wording, pagination, and arrangement can be extremely frustrating. Unless you want to have a copy for home study and prayer, there is no need to buy a prayer book: most synagogues provide them for worshippers. However, if you want to take it home or put marks in it, buy your own!
Liturgist and Rabbi Jakob Petuchowski once described the siddur as “the journal of the Jewish People.” Torah is God’s gift to Israel, but the siddur is in the words of our ancestors, our scholars, and our poets.