There are many causes of vertigo. Your perceived position in space is determined roughly by three major systems: your vision, your muscles/nerve input to your brain, and your inner ear/nerve input to your brain. Disruption in any of these systems can cause you to have problems with your balance.
Humans perceive three dimensions because we have three semi-circular canals in our inner ear. These canals are filled with fluid and have tiny sensory hairs. As your body accelerates, decelerates or changes position, the speed or direction of the fluid changes accordingly. Those changes are picked up by your nerves and sent to your brain.
Vertigo is a feeling that you are spinning, or that the room is spinning. There are many types of vertigo, but the type most commonly treated by physical therapists is called benign positional paroxysmal vertigo, or BPPV. In BPPV, tiny crystals get stuck in one of your semi-circular canals. When you change position, the fluid initially moves relative to your position in space, but then hits the crystal that is lodged and bounces back in the opposite direction. This gives your brain the message that you changed position again, when you continue to move in the same direction. BPPV is caused by a mismatch between the information your brain receives from your vestibular system, or inner ear, and the information it receives from your muscular and visual system.
Physical therapists treat BPPV by putting the patient through a number of position changes, called the Epley maneuver, which attempt to dislodge the crystals that are stuck in the semi-circular canal. Physical therapy treatment is non-invasive and does not require you to take medication. Some minor positional restrictions are recommended for a few days to one week following physical therapy treatment.
There are many other types & causes of vertigo & imbalance. In non-BPPV cases, physical therapy usually focuses on challenging and strengthening the other sensory areas that have not been affected by the cause of your vertigo.