Restless legs syndrome, also known as Willis-Ekbom disease, is a common condition of the nervous system that causes an overwhelming and irresistible urge to move the legs. Estimates are that around 10% of people suffer from RLS at some stage in their lives. It often occurs during sleeping hours and can cause severe insomnia. In the majority of cases, there's no obvious cause of restless legs syndrome (i.e. it's termed idiopathic).
RLS can be triggered (or exacerbated) by other medical problems, pregnancy, or by medications, drugs, etc.
Further reading at NHS, UK