Daleks are fictional arch-enemies of Doctor Who. They are said to have no empathy and a bottomless will to EXTERMINATE! all other life forms.
In their natural form, they are not so much of a threat: tiny, weak, without definite limbs. What makes them so dangerous is their battle armors, almost impenetrable and augmented with Dalek advance technology.
Time and again, the Doctor thwarted the Daleks's plans, to the point he earned the nickname of "The Coming Storm". The Doctor is public enemy #1 to the Daleks.