Runes of this kind carry the power of the divine or demonic source for the runes. The runes derive their power from not only the entity which drew them but the plane them come from (demonic or divine). Each set of runes carries its own properties and impact on mortals. Since the runes are so powerful the impact on mortals is rarely beneficial and as such mortals are never meant to see them directly. In all cases of such runes, even photos of the runes can cause the effects of seeing them first hand, if diminished slightly.
Demonic
Demonic runes are known only to immortal creatures of either the demonic or divine planes, specifically they carry evil power in their very existence. For non-divine beings, such as humans, simply looking at such runes can cause madness and a connection to demonic energies, more specifically the demon or demons that wrote them. They are rarely used the way humans use writing, instead they are usually used for protection or some other form of casting. Since they are so powerful they are not often used on the material planes (such as the plane that Earth is in). Enscribing runes onto an object in the material planes will imbue that object with demonic power related to the runes.
The runes themselves are too powerful for mortals to write themselves, attempting to mimic their patterns will result in frustration. Given that the runes can cause increasing levels of insanity in mortals, this can become an obsession which is all consuming, attempting to recreate the runes that are forever burned into ones mind.
Divine
Divine runes are known only to creatures of either the divine or demonic planes. They hold a great deal of divine power with them, and are mostly used for wards of protection or warning, though they can be used for spell casting as well (it is not common though). A material object with these powerful runes will gain divine power and be a source of divine will. This will is linked first to the angel or other divine entity which created the runes, but since such entities have a strong link to god these become a conduit for Gods holy will power. Since demons do not share the strong link to a supreme being and are more independent, demonic runes do not share this conduit to a supreme will power.
Much like demonic runes, however, divine runes cannot be understood or written by mortals. Direct exposure to such runes can be both a benefit and a curse. Since they are proof of the divine, a mortal viewing such runes will automatically understand that the divine exists (see Regarding Faith for information on how faith is handled in this context). This in and of itself can be extremely affirming for a person of strong faith, making it even stronger, or can be devastating to entirely change the perception of the world for a atheist. The runes can also cause obsession with the divine, in becoming closer to and doing the will of God, even if one cannot be sure what that will is. When exposed to an object with is imbued with divine power and thus a conduit of Gods will, this impulse becomes even stronger and can act like a drug to the mind of a mortal, becoming something the mind requires in order to function properly. In such cases taking away the object can cause withdawal-like symptoms including inability to think or make decisions. It is unlikely such symptoms will directly kill a mortal, but a mortal may wish to commit suicide once the divine connection is gone.
Holy / Unholy
Not to be confused with Divine or Demonic Runes. Holy runes are neither good nor evil, however they represent runes drawn by mortals at the behest of an immortal creature native to either the divine or demonic planes. Since the mortal mind and body cannot grasp or contain the power of such runes, it cannot draw them effectively. Simply the act of trying can be destructive to the mortal in question. Divine beings rarely use a mortal vessel in this way, though it is possible for a willing vessel to give themselves over to be used, it is just exceptionally rare for a divine entity to ask for such a thing. Demons, however, far more often use mortal vessels through possession into using such runes. When a demon is angry or otherwise upset and trapped in a mortal vessel, they will often vent their anger and frustration through forcing the mortal to write in such runes and thus further punish the mortals mind.
Such runes when seen by another mortal will be forever etched in that humans mind, much like their stronger cousins, but will not cause insanity or other deleterious effects except through close study for long periods of time. They also will automatically feel different to look at, Unholy (Demonic) Runes will feel wrong and awful, like nails on a chalkboard while Holy Divine Runes will feel peaceful and powerful to look at, comforting. In either case the longer a mortal studies the runes the closer the effects get to the effects of divine or demonic runes themselves, years of study can drive a mortal insane from looking Unholy Runes the same as a few seconds of looking at fully Demonic Runes can do.
Holy runes are nearly unreadable even to a being from one of the two planes, because they are what the mortal body can manifest of far more powerful runes, they are akin to a child trying to scribble something in cursive as fast as they can without really knowing more than the basic forms of letters. Difficult but somehow not quite impossible to read. A mortal can never really understand the runes, nor will they be able to reproduce them from memory - even though the memory will never go away or fade from one's mind.