Many scholarly and academic attempts to define or describe religion can be classified into one of two types: functional or substantive. Each represents a very distinct perspective on the nature of function of religion. Although it is possible for a person to accept both types as valid, in reality most people will tend to focus on one type to the exclusion of the other.
Which type a person focuses on can tell a lot about what they think of religion and how they perceive religion in human life. For those who focus upon substantive or essentialist definitions, religion is all about content: if you believe certain types of things you have a religion while if you dont believe them, you dont have a religion. Examples include belief in gods, belief in spirits, or belief in something known as the sacred.
Accepting a substantive definition of religion means looking at religion as simply a type of philosophy, a system of bizarre beliefs, or perhaps just a primitive understanding of nature and reality. From the substantive or essentialist perspective, religion originated and survives as a speculative enterprise which is all about trying to understand ourselves or our world and really has nothing to do with our social or psychological lives.
For those who focus on functionalist definitions, religion is all about what it does: if your belief system plays some particular role either in your social life, in your society, or in your psychological life, then it is a religion; otherwise, its something else (like a philosophy). Examples of functionalist definitions include describing religion as something which binds together a community or which alleviates a persons fear of mortality.
Accepting such functionalist descriptions results in a radically different understanding of the origin and nature of religion when compared to substantive definitions. From the functionalist perspective, religion doesnt exist to explain our world but rather to help us survive in the world, whether by binding us together socially or by supporting us psychologically and emotionally. Rituals, for example, exist to bring us all together as a unit or to preserve our sanity in a chaotic world.
The definition of religion used on this site doesnt focus on either functionalist or essentialist perspectives of religion; instead, it attempts to incorporate both the types of beliefs and the types of functions which religion often has. So why spend so much time explaining and discussing these types of definitions?
Even if we dont use a specifically functionalist or essentialist definition here, it remains true that such definitions can offer interesting ways to look at religion, causing us to focus on some aspect which we might have otherwise ignored. It is necessary to understand why each is valid in order to better understand why neither is superior to the other. Finally, because so many books on religion tend to actually prefer one type of definition over another, understanding what they are can provide a clearer view of authors biases and assumptions.