A touch screen display is usually made with three main parts. The protective glass sheet, the digitizer sheet, and the LCD. The digitizer is a thin transparent sheet that is adhered to the protective glass sheet with a transparent conductive adhesive. The digitizer is the only part of the display assembly that is responsible for sensing touch. Since the human touch is conductive it allows the screen to sense the location you touch. The LCD is only responsible for displaying the image on the screen. If the screen is not responding to touch then usually the digitizer is damaged or faulty. Usually a damaged or cracked LCD will display a distorted image or no image at all. So if the phone displays a good image but does not respond to touch then you may have the option of only replacing the digitizer. It's not as easy of a repair as replacing the whole display but is cheaper. A new digitizer part already has the digitizer sheet adhered to a protective sheet of glass. If a person can remove the faulty digitizer from the phone without damaging the LCD then it is surely more cost effective repair. Some displays have the digitizer fused to the LCD which makes it hard not to damage the LCD if trying to remove the digitizer. If that is the case with your phone's display it is a lot easier to just replace the whole display. Hope this information is helpful.