Lighting not only lights up houses and other lots when it's dark, but they also have an effect on the Environment score.
All lights work the same, whether they're table lamps or ceiling lamps. They all have the same interactions. Sims can turn lights on and off, but more specifically, they can turn on/off the light they've selected, turn on/off all the lights in the room, or turn on/off all the lights on the lot. Another interaction is to activate Auto Lights, which means that lights only turn on when a Sim is in the room. When they leave, the lights will automatically turn off. Like the Turn On/Off interaction, Sims can activate this feature with the light they've selected, all lights in the room, or all lights on the lot. The good thing about interacting with lights is that your Sims don't have to walk right up to them to carry out the interaction.
This page covers outdoor lamps. Despite the title, they don't necessarily have to be placed outside, but some of them can only be placed on a fence post.
