Whats a ballast for?

Ive seen several 400w hps fixtures that just have a 120v cord attached directly to the light. can anyone explain what the ballast does? and why do some lights need them?

its essentially just like an amplifier

so the light will work fine without it?

A ballast ensures a few things, first it regulates the pulse on pulse start type bulbs. Because connecting a lamp directly to a socket would not work it would spike the energy to the socket from the pulses which of course is not good. What you have most likely seen is the lamp socket with 120 volt cord, and the cord hooks to remote ballast. You've got to have a ballast for HID lights.