Please help! what is the difference between pleural effusion and thoracentesis?

Difference...: Pleural effusion means there is fluid present in the space surrounding the lung. A thoracentesis is a procedure where a needle is placed through the skin and into the pleural space to remove a sample of the fluid surrounding the lung. An ultrasound or ct scan can determine where there is the greatest amount of fluid so this is usually done just prior to the thoracentesis procedure.

They're related: Pleural effusion: fluid in the space around your lung thoracentesis: procedure in which an effusion is drained by placing a catheter between the ribs into the space around the lung.