Headlamp

LEDs
LED stands for light-emitting diode. LEDs are technically not a bulb but a semi-conductor. LEDs provide dim light for 20-100 hours. Due to the low amount of light provided by LEDs, they are often used in sets of 2 to 6 LEDs. Most headlamps that use sets of LEDs permit you to regulate how many of the bulbs are illuminated at one time. This allows you to save battery power when a bright light is not required. S-LED stands for super LEDs. S-LEDs are considerably brighter than regular LEDs. Some, like the ones on the Petzl Myo XP require 3 watts of power and are approaching the same brightness levels of traditional bulb lights.

When selecting an LED light also look for a battery regulation feature. In an unregulated LED light, the light's power decays exponentially with a loss in battery power. If you look at headlamps such as Petzl's Myo XP, Myo 5 or Princeton Tech's Apex, their lights have an extended curve that allows for more usuable light for a longer period of time.

Bulb
When selecting the right bulb consider how long you want the light to last. Typically the brighter the bulb, the more power they consume.

Halogen - provides a bright but yellow toned light. A halogen lamp is an incandescent lamp where a tungsten filament is sealed into a compact transparent envelope filled with an inert gas, plus a small amount of halogen such as iodine or bromine. The halogen cycle prevents darkening of the bulb. The halogen lamp can operate its filament at a higher temperature than in a standard gas filled lamp of similar wattage without loss of operating life. This gives it a higher efficacy (10-30%). It also gives light of a higher color temperature compared to a non-halogen incandescent lamp.

Krypton - Provides a whiter light than halogen.

Xenon - Colorless, orderless, gas used in a gas discharge lamp to produce white light. Brighter than Halogen, Krypton and LEDs for less power than a halogen.