What are the main components of a HID kit?

Typically a HID kit has the following components.

1- Ballast.

Think of this as a step up transformer that takes your cars 12v power source and increases the voltage up to 23,000v. This high voltage is crucial in order for the HID bulb to work.

Every ballast will require a 12v input from the cars original bulb wiring and will      also have an output lead which will connect to the HID bulbs.

In a kit there are typically two ballasts (one for each headlamp).

2 - Bulbs.

Each ballast will ultimately connect to a bulb. The HID bulbs have high tension leads that connect to the ballast 

3 - Wiring / Fixtures.

Additional wiring (relay) for bi-xenon bulbs (H4 where both the dip beam and high beam is in one bulb).