Many users are confused on what is digitizer, screen or LCD, LED. If your screen is cracked or damaged but still able to use it or see it, your LCD/LED (picture displaying unit) is fine, then you will need a digitizer / screen replaced only. LCD (Liquid Crystal Display) is the one displays the picture and images.
Here are the examples.
As shown in the picture above, the screen is still functioning. However due to different manufacture process, some phone might not work even though the display is functioning 100%. (i.e, LG NEXUS models.) Then you will have to replace the digitizer. Some keeps using the phone as it is still working, but the cracked glass keeps pressing down, the LCD will eventually will get damaged which then looks like the picture below (called bleeding). When this happens, you will have to replace the glass and the LCD together which will cost more.
we have added new video describing the components. Check it out here.