If you want to convert your halogen headlight to a HID xenon headlight you need a ballast in order to make this work. Due to the higher amount of power needed to make a HID xenon bulb light up, a ballast is necessary. This is the power unit that powers a xenon bulb. You cannot use xenon bulbs without a ballast. You can however install projectors, but still use your OEM halogen bulbs. In this case you don't need to install a ballast. This will still improve your light output and light pattern in comparison to  your OEM halogen headlights. But please note, the quality and the amount of light will still be significantly lower then when you use xenon bulbs.