Ballasts:

Ballasts are used to regulate the energy flow to light bulbs. If we were to run a light without a ballast and plug it into the wall it would explode. This is because light bulbs have a negative resistance, this means the energy the lamp draws will become greater and greater until it explodes. (If you were to throw a coin off the empire state building it would accelerate faster and faster until it hit the ground. The reason it would not kill somebody is because of air resistance and terminal velocity, these two items are similar to a ballast in the way they limit the speed of the penny, or in our case limit the current supplied to the lamp). When you have ballast for a 96 watt lamp, this means the ballast cuts the power from your wall outlet down to something usable by the 96 watt lamp but not so much that it would explode. If you plugged a 10 watt lamp into the 96 watt ballast it would explode because the ballast would be supplying it with enough power for a 96 watt lamp to a 10 watt lamp.

This is a ballast with an enclosure. This particular ballast is for 150 Watt HQI lamps.

Replacement:
Ballasts should be replaced when the lamp will light in another fixture but not the fixture with the suspected faulty ballast. Sometimes lamps can take down ballasts when they burn themselves out. Other times, and most times, a lamp will burn out when the ballast fails as well.
Some ballasts have quick disconnect ends that allow for easy setup, some are just a raw ballast and have to be manually wired into the fixture for replacement.
Electronic ballasts may power different type of bulbs (such as T5 HO, NO, and VHO in one ballast) and should be carefully researched on websites including the manufacturers website to ensure it is the ballast you need.

Identification:
Oftentimes ballasts will contain an ANSI code. This code is registered with an organization called the American National Standard Institute. These codes are made to ensure items with the similar code will all perform and are characterized in similar ways. If your old ballast has an ANSI code of M143, different branded ballasts with the same ANSI code will perform and work similar to your old ballast.