That is a decision that has to be made individually but usually the overhead lights in rooms (whether it is your house or a business), outdoor lighting that does not impact safety, decorative lights, neon signs for advertising, televisions, desk lamps, the list goes on and on.
There are a few lights we can say with certainty that should NOT be turned off, including safety lights in public spaces, lights for aviation guidance, traffic lights, security lights, just to name a few. We ask people to use common sense. Before you turn off any lights for public spaces, Earth Hour recommends you check with local officials or community centres.
In your own home, use common sense with respect to safety. Keep small night lights on for basic safety especially in halls and on stairs. Make sure you have alternative light sources handy before Earth Hour starts, like torches or flashlights. That way if you need to see, you have a light source close at hand, and you can still respect the spirit of Earth Hour and keep yourself and your family safe.