What is the difference between atelectasis and collapsed lung?

Not much: Collapsed lung is called atelectasis which could be secondary to pneumothorax, mucous plug or tum etc or infections, .

Lung collapse: It is one in the same, just a difference in degree. Atelectasis is use to describe more limited areas of collapse but can involve segment, lobes, or even the entire lung.

Grades of the same: Atelectasis (from the latin for imperfect extension) can refer to collapse of a small portion of the lung (called a segment or subsegment) or to collapse of an entire lung. It can be due to obstruction of an airway. Depending on the level of the airway (whether a main bronchus or a smaller bronchiole) depends on the amount of lung that collapses.