When buying a headlight assembly, you would buy them for each side of the vehicle – either a right or left assembly as they are not usually sold in pairs. Whereas headlight sealed beams or bulbs are sold either individually or in pairs.
There are many different types of headlights for many different types of vehicles, so depending on the year of the vehicle that you have, it may have:
Sealed beam headlights
Replicable halogen bulbs
Sealed beam headlights are more often used on older vehicles and they are the dimmest of all the headlights that you can get. If you upgrade to halogen headlights you will be able to get more light in many different styles from:
But it will all depend on what headlight bulbs you decide to get.
The best headlight is a HID or a High Intensity Discharge headlight. These can sometimes also be called Xenon headlights. These are the brightest lights and they will light up the most area that you can get. You will also find that HID headlights will usually have either a purple or a bluish tint to them.