Any injury caused by an inequality of pressure. This is a particular problem in scuba diving since pressures can vary so greatly between air spaces in the body (such as lungs, middle ear, and sinus passages) and the surrounding water. It can result from either an expansion of the space during an ascent or compression of the space during a descent.
Pulmonary barotrauma involves the lungs, usually due to expanding air during an ascent. In other words: if you hold your breath while ascending, the air in your lungs will expand and your lungs will rupture. It's as scary as it sounds. An air bubble entering the blood stream, known as an arterial gas embolism (or AGE), can cause death.
Ear squeeze is simply due to pressure on the air inside the Eustachian tube of the middle ear during a descent. This can be counteracted by equalization: pinch your nose and either blow against it or swallow. This forces the compressed air in your lungs outwards through the Eustachian tubes. On ascent, this is known as a reverse ear squeeze.
Sinus squeeze is caused by blocked sinus passages during a descent. On ascent, this is known as a reverse squeeze.
Abdominal squeeze is caused by gas in the intestine that may expand while during ascent. It can easily be relieved through flatulence.
Mask squeeze is caused by the low pressure on the part of the face covered by the mask. This may result in bruising or swelling around the eyes.