The D2 bulbs are OEM HIDs. They are used for headlights mostly, though there are cases when one D2 bulb may perform for both high and low beams (2009-2010 Audi A8 and 2003-2008 Infinity FX45) due to movable housings. The D2 and D4 HID bulbs are designed in the same way, though they cannot be replaced with each other. The D2 bulbs cannot be used with D4 ballasts and vice versa. Kensun offers D2R and D2S kits which are available in different colors. The OEM replacement bulbs can be found also. They can be used with factory D2 ballasts.
In case you would like to try something different from HIDs, have a look at Kensun LED kit. It does not require ballasts that is why they should be removed.
In the case of upgrading of the whole HID system or converting to LEDs, professional installation may be needed.