Disintegration is a way items and creatures can be destroyed. Disintegration is one kind of instadeath, which occurs when you are hit by a blast of disintegration. The only possible sources of such a blast are a black dragon's breath or a particular flavor of your God's wrath - wide-angle disintegration beam (which can also destroy worn armor, helmets and shields, even if you are reflective or disintegration-resistant); there are no spells or wands of disintegration. Magic resistance does not save you from disintegration.
Reflection will reflect a blast of disintegration, but not a wide angle disintegration beam. You can gain disintegration resistance, by wearing black dragon scales, black dragon scale mail or by eating a black dragon corpse (this will protect you from both sources of disintegration).
Certain items can also disintegrate; a spellbook has a chance to disintegrate when you fail to read it. An uncursed scroll of scare monster disintegrates when picked up if it both is uncursed and already has been picked up once, and a cursed one always disintegrates when picked up.