An aurora is a bright glow that occurs in the night sky when energetic particles, mostly electrons, enter the Earth's upper atmosphere from the magnetosphere. When they break through, they collide with atoms and molecules, which take some of the energy and store it, creating what's known as an excited atom. The only way to calm this atom down is for it to rid itself of the energy by firing off a photon. This makes the glow we see here on Earth.
There are all kinds of auroras, but the most well-known are the aurora borealis, or Northern Lights. These are the multi-colored lights in the Northern Hemisphere's polar region. Green, white, purple and red glowing "curtains" blanket the dark, clear skies of the North Pole about 1,500 times a year during what's called a substorm. This is when the sun releases hot plasma gas as energy into the magnetosphere. This gas breaks up and penetrates the Earth's atmosphere and curious humans below marvel at the breathtaking result. For your best chance at witnessing the Northern Lights, trvael north during the winter in Canada, Alaska or Scandinavia and then keep and eye on the skies.