They are brown with round bodies and short, cartoonish arms and legs. They are around 49 cm tall and scamper around in a busy, cute and flat-footed manner. They bounce like rubber. They have huge dark eyes. They can lift in the region of thousands of times their body weight.
Imps are not demons or related species. They are manifestations of hell's 'mind' and act as servitors, drones, repair mechanisms or antibodies. They occasionally make chittering and squeaking noises; they have never been reliably reported to talk or pay any attention to passers-by - whether demon, hellhound or other. It is rumoured that the demon king can control them, but that isn't the case.
Imps are one of the most obvious throwbacks to one of Mutt's cardinal formative influences: the Bullfrog game Dungeon Keeper.