What's the difference between the iPad Digitizer and LCD?
The iPad's front assembly is not all that different than the iPhone 3G/iPhone 3G S front assembly. It consists of a frame, an LCD screen, and a digitizer. The LCD screen on the iPad (and on iPhones for that matter) is the actual display. The digitizer is the glass on top of the LCD screen. The digitizer is what reads the touch gestures on the screen.
To try to word this an easier way, without the digitizer, touching the screen would not do anything at all besides leave fingerprints in the LCD screen. Without the LCD screen, you wouldn't be able to see anything!
And How do I know which repair service or part I need?
If your iPad's glass/digitizer is cracked or broken, you should still be able to see the screen clearly through the cracks on the outer glass/digitizer. In most cases, you should still be able to use the iPad and its touch functionality should still be intact. In other cases, the touch functionality will cease to work for no apparent reason. If this happens and you can still see the screen clearly, the digitizer is defective and needs to be replaced.
If your iPad's LCD screen is broken, the display would be either completely blank, very dim, or parts of the screen will not show anything at all while other parts of the screen display just fine. In some cases, both the digitizer and LCD screen can be broken.
The most common type of repair with the iPhone is a digitizer repair. You can expect the same with the iPad.
So, long story short, if JUST the outer glass is broken, you need a digitizer repair. If the outer glass is not broken but the display is not working correctly, you most likely need an LCD screen repair.
If you don'w know which repair you need, iResQ offers a free diagnosis for iPads. They even offer free diagnosis for iPods, iPhones, Apple Laptops and Desktops, and even the SONY PSP.