Low Key: The term "Low Key" is to refer to any scene with a high lighting ratio, especially if there is a predominance of shadowy areas. It tends to heighten the sense of alienation felt by the viewer. Mostly used in film noir or horror films.
High Key: The scene is flooded with light. This in turn leaves very few shadowy areas and create a washed out look. In order to create a high key lighting, many extremely bright lights must be used in order to flood the scene with light. It tends to be used to reveal settings, characters, objects to reveal it to the audience, often used in happy and energetic and bright/warm situations often in action, drama and comedy genres.