I was wondering what are the differences between pleural effusion and empyema?

Yes/no infection: Pleural effusion is fluid in the space around but outside the lung (s). It could be reactive or is subdivided to transudative or exudative. The exudative, once frank pus from infection is seen is called empyema.

Pus/fluid: Empyema is pus in the chest or a purulent effusion and an effusion is a fluid collection, may be serous, purulent, hemorrhagic, para pneumonic.