It is a phenomenon often confused with lightning and most readily observed in a situation where the ground is charged and there is a sharp point of something on which the phenomenon is observed.  It can be anything from the tip of a building to grass or even a cow horn (apparently).  The glow is actually produced by the air molecules being split apart leading to the ionization of the air molecules and the production of a small field of plasma.  The glow can be blue or white but can also be purple.  The intensity of the light is a factor of both the electrical charge present at the time and the geometry of the point from which the plasma is created.

St. Elmo's fire is an electrical phenomenon that is best known for appearing around the masts of ships back in the days of sailing ships.  It can also form around airplane wings and many other objects.

This phenomenon is caused by a great difference in electrical charge between a charged object (like the mast of a ship) and the air around it.  When the charged object's voltage gets high enough, it will discharge the electrical energy that has built up.  This discharge can be seen as a glow -- St. Elmo's fire.