A prayer book is a book containing prayers and perhaps devotional readings, for private or communal use, or in some cases, outlining the liturgy of religious services. Books containing mainly orders of religious services, or readings for them are termed "service books" or "liturgical books", and are thus not prayer-books in the strictest sense, but the term is often used very loosely. A religion's scriptures might also be considered prayer books as well.

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Prayer has many different forms. Prayer may be done privately and individually, or it may be done corporately in the presence of fellow believers. Prayer can be incorporated into a daily "thought life", in which one is in constant communication with a god. Some people pray throughout all that is happening during the day and seek guidance as the day progresses. This is actually regarded as a requirement in several Christian denominations, although enforcement is not possible nor desirable. There can be many different answers to prayer, just as there are many ways to interpret an answer to a question, if there in fact comes an answer. Some may experience audible, physical, or mental epiphanies. If indeed an answer comes, the time and place it comes is considered random. Some outward acts that sometimes accompany prayer are: anointing with oil; ringing a bell; burning incense or paper; lighting a candle or candles; facing a specific direction (i.e. towards Mecca or the East); making the sign of the cross. One less noticeable act related to prayer is fasting.
 
A variety of body postures may be assumed, often with specific meaning (mainly respect or adoration) associated with them: standing; sitting; kneeling; prostrate on the floor; eyes opened; eyes closed; hands folded or clasped; hands upraised; holding hands with others; a laying on of hands and others. Prayers may be recited from memory, read from a book of prayers, or composed spontaneously as they are prayed. They may be said, chanted, or sung.