H4 Halogen bulbs in the High / Low configuration have two filaments inside the glass, and three metal pins for connection to power (common ground + high beam positive + low beam positive). The round base also has three metal lugs to align the globe correctly in the headlight socket when installed.

One of the filaments has a small shield on one side. This is the low beam filament. The second filament has no shield, and is closer to the base of the bulb. This is the high beam filament. Each filament can range between 55 and 130 watts.

HID Hi / Lo bulbs in comparison do not have two "filaments" (actually called "chambers" since HIDs do not use a filament). Instead, HID bulbs use just the one chamber/light - but they move this backwards and forwards in the socket by use of a magnetic solenoid.

When "low beam" is on for a HID bulb, the chamber sits normally in the socket. When you engage high beam however, the solenoid is turned on, and this pulls the glass part closer to the base. In doing so changes the focal point of the beam, throwing the light further up the road (high beam). This is why Hi / Lo HID kits require an extra wiring loom that has the High / Low controller.