The roach is the end of the joint, typically consisting of the crutch and a minuscule amount of cannabis left at the end. They're usually discarded because they're so difficult to smoke, except in cases of desperation where they become the only option. You could just scrap out the last bits of flower and smoke it in a bowl instead, or use a roach clip to keep your fingers away from the fire.
This website contains all the types of cannabis.
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