what are the northern lights?

There are many different forms of aurorae, but the most well-known look like curtains of light, with ripples and curls moving along them.
This is a movie of some aurorae in action. Fantastic!

Here is a movie, taken from a spacecraft, of the ultraviolet radiation from a display of aurora around the north pole. It shows where the atoms in the Earth's atmosphere are being excited by the particles from the Sun. By studying images such as these, aurora-watchers can tell people on the ground where arorae are likely to be visible from.