Prayer is a direct and unequivocal act of relating to God.
In whatever way it is performed, and in whatever manner it is uttered, prayer is essentially one thing:
an explicit addressing by the human “I” to the Divine “Thou.”
In the most essential sense, prayer is direct speech, in which man confronts and addresses his Creator.
Such speech may be of many kinds: request, supplication, thanksgiving, complaint, or even simple conversation.
All these can be found in prayer, and each one of them can be expressed by personal, individual prayer.
The prayers that may be found in the books of the Bible-particularly in the Book of Psalms, which is basically a compendium of individual and public prayers-represent all of the kinds and varieties of prayer with which the individual or public may address God the Creator.
The wide range of prayers and benedictions found in the Siddur likewise include the entire spectrum of ways in which a man may address his Maker.
–Rabbi Adin Steinsaltz