The Northern Lights is an amazing natural wonder that can only be seen in the far northern hemisphere (near the Arctic Circle) in the winter. Even then, there is no guarantee that it will be seen. The sky must be clear and dark.
The only way to describe it is that it is like a green flame slowly flickering away in the sky.
We went on the night tour twice. On the first day, we were not so lucky. After braving strong winds at -7 degree Celsius, we could only catch a glimpse of it. On that day, it looked liked some dark greenish clouds in the sky, nothing fantastic or amazing. We were offered a discount to go on the tour the next night, which we did.
The second time around we were much more fortunate. Immediately outside of the city, away from the city lights, we saw the Northern Lights! The coach brought us to a hotel in a National Park (Pingvillir National Park, I think) and we used it as a base. We were at the car park watching the lights and running back into the hotel to get warm!
The lights display was amazing -- slowly transforming itself in its shape, brightness, and intensity. There is nothing in the world that can describe what it's like. It covered the entire stretch of the night sky. Then it slowly became horizontal across the sky, turning to become a vertical streak. At one point in time, it spread out from the top of sky in multiple directions. Words cannot fully describe what it was like.