Philosophy of Religion is the branch of philosophy that is concerned with the philosophical study of religion, including arguments over the nature and existence of God, religious language, miracles, prayer, the problem of evil, and the relationship between religion and other value-systems such as science and ethics. It is often regarded as a part of Metaphysics, especially insofar as it is interested in understanding what it is for something to exist, although arguably it also touches on issues commonly dealt with in Epistemology, Ethics, Logic and the Philosophy of Language.
It asks such questions as "Are there sound reasons to think that God does (or does not) exist?", "If there is a God, then what is he like?", "What, if anything, would give us good reason to believe that a miracle has occurred?", "What is the relationship between faith and reason?", "Does petitionary prayer make sense?"
It does not ask "What is God?", as that would assume the existence of God, and that God has a knowable nature, which is more the territory of theology (which usually considers the existence of God as axiomatic, or self-evident, and merely seeks to justify or support religious claims).