Hoarding disorder is a persistent difficulty discarding or parting with possessions because of a perceived need to save them. A person with hoarding disorder experiences distress at the thought of getting rid of the items. Excessive accumulation of items, regardless of actual value, occurs.

Source : Mayo Clinic, US

It's estimated to affect between 2 - 6% of the general population. The causes of hoarding disorder are unknown. Currently studied factors include, genetics, brain function disturbances and stress. It's also often 'associated with' alcohol dependence.