A full HDTV is a television that has can display images and movies at 1920 by 1080 pixels resolution. In comparison, a HDTV can only display 1280 by 720 pixels.
Every frame of a television display consists of small dots, called pixels. Because the pixels are so small, viewers only see the image that the pixels form. The full HDTV resolution means that the television can display 1920 pixels horizontally and 1080 pixels vertically. The image quality is directly proportional to pixel count. However, televisions only perform optimally at their native resolutions. As such, a full HDTV cannot display a 1280 by 720 pixels image as well as a HDTV can.