"What exactly is it?"
Shatter, like wax, oil, crumble, sap, is a nickname for a different type of cannabis extract. Shatter has a reputation for being the purest and cleanest type of extract. Shatter is made by saturating marijuana with butane and evaporating off the butane. What remains is the resin. Shatter is consumed by 'dabbing' or the act of smoking cannabis concentrates using an apparatus that heats the resin for inhalation.