Paracentesis refers to a procedure to remove excess fluid via a needle syringe from a body cavity, most often the abdomen. Removal of fluid from the chest cavity is called thoracentesis. The causes of fluid buildup in the body are many, including liver disease, congestive heart failure, pneumonia, infections, and certain cancers. Draining the fluid relieves pressure on the patient's internal organs which may cause more serious complications and provides a sample for diagnostic tests.

Performed on an outpatient basis under local anesthesia, doctors insert a hollow needle with a plastic sheath into the abdomen, chest or affected area. This is done with the guidance of ultrasound imaging that helps doctors place the needle at the optimal location and depth and avoid other organs in doing so. Once in place, the doctor withdraws the needle, leaving the plastic sheath as an outlet for fluid drainage.

Though very safe and generally painless, these procedures carry a very small risk of infection or bleeding. Afterwards, patients can return to normal actitivities. 

For more information on thoracentesis:

http://www.nlm.nih.gov/medlineplus/ency/article/003420.htm