Usually in the shape of balls, ghost lights (also called spook lights or earth  lights), are patches of light that appear randomly or in patterns at particular sites. They vary in size and may be active for short periods of time or for years. Most are white or yellow, but some have been reported as red, orange or green. They may change colors while being observed. Some reports of ghost lights can be explained naturally, for example car headlights or ionized gas escaping from faults in the earth. Ghost lights have been researched by the Ghost Research Society and their findings are that:

(1) They are usually seen in remote areas
(2) They can only been seen from certain angles and distances
(3) Loud noises or bright lights may cause them to disappear
(4) Sometimes they are accompanied by a humming sound
(5) They are associated with hauntings involving accidents or tragedies involving loss of life

Some of the most famous ghost lights are the ghost lights at Marfa Texas first reported in 1883. These lights bounce up and down and move across the ground like grassfire. Witnesses here have said the lights seem to be "playful".

More about the Marfa Lights:  http://www.watchingyou.com/marfa.html
The Lights at Cohoke:  http://pointer.wphs.wpps.k12.va.us/co_eye.htm
The Surrency Spook Light: http://www.strangemag.com/surrencyspooklight.html

There are also several personal accounts on the Shadowlands. Read one of them here: http://members.aol.com/shadoland2/ghost3.html#ghost light