There are many different styles and models of emergency lights available, but they all basically do two things.
They can be used as a night light. Some older models have an orange neon light; while others have a soft green glow. Technically they are more guide lights then night lights as they don't really illuminate a room like a night light would. What they have in common is they use a regular bulb for their flashlight when the power goes out.
Some newer lights use LEDs. It has an LED for the night light and 3 LEDs come on for its flashlight function. They all plug into a wall outlet and when the power goes out you unplug them to use for their flashlight function.
Something else to consider is whether to have emergency lights that use regular replaceable batteries or rechargeable batteries. The advantage of rechargeable is they are ready to use as they are always on-charge. On the other side during a long power outage once the batteries are depleted the light can't be used until it can be recharged. No matter which type decided on they are typically safer than candles.
A quick side note; LEDs are beginning to be seen as replacements for standard lights. Compact fluorescents use about 80 percent less energy than an incandescent light bulb. The new LED bulbs use around 80 percent less power than a compact fluorescent - but in my opinion that LED bulbs aren't ready for prime time. First, just like CFL bulbs they don't always fit where a regular bulb was designed to go.
Some bulbs are not to be used in a light with a shade - the light output just isn't enough and it doesn't glow in all directions. Don't get me wrong. LEDs will be replacing other types of bulbs as the future unfolds.