The Hebrew word for prayerbook is Siddur, which comes from a Hebrew root (Samech-Dalet-Resh) meaning "order." The Siddur, then, is a book that sets forth certain Hebrew prayers in a very specific (time-based) order. There are many types of Siddurim available, but they all share a similar underlying structure.
Observant Jews pray in formal prayer services three times a day, every day (corresponding to the sacrifices once offered at the Temple). Accordingly, most (weekday) Siddurim include Tefillat Shacharit (morning prayers), Tefillat Minchah (afternoon prayers), and Tefillat Ma'ariv (evening prayers). In each case, one prayer - the Shemoneh Esrei (also known as the Amidah) - is recited. The last prayer to be said each day is the Keri'at Shema al hamitah (the bedtime Shema). Shabbat Siddurim are ordered in accordance with the routine of Shabbat. Since it begins in the evening, the first set of prayers concern candle lighting and welcoming the Shabbat.