First, consider rapid start and programmed rapid start as similar technologies. The first is used by older magnetic ballasts while the latter is a more refined version used by modern electronic ballasts. Both types effectively ramp up the voltage on a lamp to light it. This is a relatively "gentle" approach to getting it lit, and preserves the rated life of the lamp. It doesn't provide an "instant on" experience, but still provides light soon after the switch is turned on. These types of ballasts can be used in areas using occupancy sensors, or anywhere lights are frequently turned on and off.
Instant start ballast technology sends a more powerful, immediate surge of voltage to provide immediate light. But the power of the surge is a jolt to the lamp and can shorten its rated life if used too often, so this technology is generally used where lights will not be turned on and off frequently. Once the lights are on, this technology is more efficient than the other two, so it helps to reduce energy costs.
This video explains these points and a few more: