High Key and High contrast are lighting descriptive terms in photography that describe the use of bright light in a photographic piece.  Often confused by new photographer and photography enthusiast thinking these are the same but in reality is a great different approach to the use of high intensity lighting in image making. High contrast is characterized by marked disparity between the main and fill light that creates an image with a very distinctive density range. In the other hand a high key image presents a low density range in the light tones with minimal or faded mid towns or shadows and the absence of bright and dark tones with the exception of a few ascent or defining elements.  The following photos illustrate the difference. Notice the absents of shadows and the relatively similar in intensity light and medium tons in the first image, this is a high key photo. In the second image you can see the distinctive disparity on tones and the marked shadows characteristic of High Contrast images. 
 

High Key Image