PFT is called pulmonary function test and is comprised of many tests, most important of them is spirometry. Other test that are done sometimes includes plethysomography, thoracic gas volume, diffusion lung capacity, pulse oximetry, arterial blood gas (ABG) etc.
Spirometry is a test designed to measure lung capacity; i.e. amount of air one can breathe out in one second. It is very helpful in evaluating asthma, COPD and other causes of obstruction to air flow i.e. vocal cord dysfunction or other diseases of trachea.