Demon Spawn are not "demons" exactly. They are a human or animal that has been shifted closer to demon through an infusion of demonic energy. They are used as disposable fodder, and for entertainment, by actual demons. 

Much much stronger than a regular human, they are grotesque and warped figures. They can only use physical attacks, and have very strong physical defenses. However since they cannot use magic, they are susceptible to magical attacks, and can be put down with adequate firepower (and yes, literally using magical fire is a decent choice).

If someone tells you that they have "slain a demon", this is often what they have killed. While very impressive, please do not think you can storm a gate to the demonic planes if you have managed to slay a spawn.

-The Adventurers Guide to the Continent