What are the lumpenproletariat?

Seriously, I am unable to really find much about them outside of short lines from Marx and Mao about their potential destructiveness among other things, but I still do not really know what that "class" is. It seems to refer to the poorest members of society that includes unemployed, criminals, homeless, etc.. And are they really so incapable of being utilized in revolutionary activities as they are portrayed?

Edit: By "destructiveness", I refer to how Marx and Mao portrayed them as people that are not considered reliable allies in any proletarian revolution (though even this understanding might be wrong because I think the explanations about them are vague).